Maybe it's just because my intro to potterslash was the first draft of a story entered to the Snape Fuh-Q Fest at Kardasi.com, where Snape was a servant of Voldemort all along, and claims Harry as his prize after the war. Maybe it's because I've been in a situation similar to H/S; as a virgin teen, I seduced the much-older game master at our weekly D&D game. Maybe I just have a thing for cynical, bitter men… or a thing for Alan Rickman, which transfers to his characters. Whatever the reasons, HP/SS is my favorite Potter 'ship, both in the stories and in fanfic.
It seems like such an obviously intriguing pairing to me, so much so that I was surprised to find out it was once (possibly still is) considered a rare or exotic relationship for slash. It resonates with the classic Kirk/Spock dynamic: younger character, brash & heroic, is forced by circumstance to cooperate with an older man who keeps his emotions strictly in check. Of course, K/S never had the overt hostility of HP/SS, nor was the more mature man in Trek in the position of authority. These are intriguing challenges for the author, who must shift "hostility" to "sexual tension", and find a way around both the laws of the wizarding world and the common aversions to cross-generational, sometimes underage, relationships.
Skipping over the first (I'll get back to hostility-into-sex later), I'm fascinated with the legal/moral issues involved. We know from canon that Harry is near-oblivious to wizarding law; he knows little of the particulars, and cheerfully ignores any rule he thinks is too restrictive. He has all the blithe confidence of any teenager—he's firmly convinced of his own ethical foundation (knows he's a "good guy"), and believes any rule or law he feels like breaking is meant for other people—meant to stop "bad people," not those with noble motives.
The Forbidden Forest is off-limits because it's dangerous… but since he feels ready to face those dangers, it's okay for him to venture inside. Wandering the grounds after curfew is against the rules… but he only breaks this rule for "a really good reason." (Note that he never checks with an adult to find out if the reason is good enough. Harry doesn't trust the judgement of most (any?) adults.) He avoids magic use at the Dursleys'… but only because he'll be punished, possibly expelled, if he doesn't, not because he thinks it's wrong for him to use magic at will.
This may be the key to Harry's approach to rules: because of his neglectful, bordering on abusive, childhood, he thinks of rules as being arbitrary constructs set in place to promote the interests of their creators. He knows those interests may be fine and well-intentioned, but believes even those can be misplaced, as the creators of the rules didn't know about *his* situation, *these* circumstances, and therefore he doesn't need to follow them. He seems to have no notion of a society of law, where all the people have agreed to follow rules (even if they chafe sometimes, or restrict them unnecessarily), in order to create something like safety and justice for all. He understands "rules that stop Bad People from doing Bad Things," but not rules for the purpose of creating an orderly society in which people can live comfortably, even if that means some people are punished unfairly.
Snape, on the other hand, does not lack this awareness. He has a keen understanding of the value of rules and law, perhaps because he's spent so much time on the other side of them. He hates that Harry breaks the rules, hates that Dumbledore & McGonnegal laud him for what he accomplishes while doing so. He feels they are teaching Harry that the ends justify the means—a dangerous lesson for the young, who don't have the foresight to understand the long-term consequences of their actions, the true "ends" they are working toward.
He understands that point far too well. Snape acts like someone guilty about his history; we don't know why he stopped being a Death Eater, but we know Dumbledore trusts him—and so do the Weasley parents, Lupin, Tonks, Moody & other members of the OotP. They don't seem to *like* him very much (let's face it, he's just not a likable guy), but they don't question his loyalty, only his hostility toward the children.
One possible reason for Snape's insistence on following rules is his own lack of understanding the "rightness" or "ethics." Like Harry, we're shown that he grew up in an abusive household, but apparently, instead of Harry's innate sense of fairness, we're led to believe he developed a selfish, "whatever I can get away with" approach to life. Or perhaps they have exactly the *same* core ethics… but Harry's goals are simple and fairly innocuous—he wants to be liked, and to be a better person than those who hurt him—while Snape's are darker. (Or they were darker. He seems to want to atone for his past.) We aren't shown anything that directly indicates Snape's motivations, be they power, glory, or revenge, but we know that Voldemort has always been a callous, manipulative bastard, willing to commit any atrocity for his cause, and nobody motivated by kindness or nobility would bind himself to such a man.
We can infer that neither Harry nor Snape gives a damn about laws qua laws; both believe laws exist for some purpose. They don't want to get caught or punished for breaking the rules... but not because they have aversions to being rebels or criminals, only because of the consequences it can bring. Snape shows no sense of fairness or "justice;" he regularly favors his Slytherin students; Harry shows no sense of respect for the wisdom of those who developed the rules he's supposed to follow.
Snape has committed horrors (or at least, seen them & not stopped them), and we know he's now devoted to stopping those horrors. We can assume there's a good deal of guilt in his bitterness, and that part of why he's so harsh to the students is to remind them—and himself—that he is Not A Nice Guy, not worthy of respect, friendship or love.
With that in mind, his connection to Harry is complex: He despised James Potter, but that doesn't mean he wanted him dead—and Harry is a constant reminder of Snape's bad choices and the unpleasant situations that drove him to them. The son of his childhood rival, who was killed by his former master… Snape would like to believe that Harry is just like his father, so he can believe that some or all of Harry's suffering has been justified.
He'd like to hate Harry. Maybe he even succeeds. But he can't deny that Harry is noble; he consoles himself (and warns others) that Harry is ill-informed and prone to leap to conclusions, and will act without thought of consequences. It frustrates him that Harry's inconsiderate impulsiveness often has good and useful results—OotP is the first book where Harry's "I gotta fix it NOW" approach has caused irrevocable damage to someone he cared about. Snape must be exulting with vindication, even as he's worried about the effect this will have on Harry—even if he's got no sympathy at all, he knows it's important for Harry to grow into his full power. And we'd like to believe he's at least a bit sympathetic; after all, Snape knows what it's like to regret choices made without looking ahead.
All these emotions, all these convolutions, plus Snape's naturally acerbic nature, bring out a palpable hostility that throbs with possibilities.
Even the common name for this pairing—Snarry—is filled with implications. A snare is a trap, and any good Harry Potter/Severus Snape story includes a feeling of being trapped, being caught in a tangle of circumstance and emotions. I've heard some people advocate the label "Snotter" from Snape/Potter, but I disagree. While that does carry an unpleasant, almost squicky connotation that's often very appropriate, it puts their names on an equivalent basis. Snarry helps remind people of the *un*equal dynamic that exists between these two characters; in canon, they do not meet as equals, and this carries over into most of the fic.
Usually, Snape is in a position of authority: either Harry's teacher, as in the books, or the superior soldier in the war against Voldemort, or an experienced teacher as Harry returns to Hogwarts to teach after the war, or in the darker fics, as a Death Eater who holds Harry prisoner. In a few, Harry is the dominant character—the successful hero who takes the broken ex-dark minion under his wing, or the charming, gregarious seducer making his moves on the insecure introvert. There are few stories where they truly meet as equals ("Nick In Time" comes to mind), but even in those, the "trapped" feeling permeates the story, and the ever-present hostility leads to plays for dominance.
We know how close anger can be to lust—that one strong emotion can jump to another. Part of Snape's job is to protect Harry, and Harry keeps getting himself almost killed by forces beyond his control. This creates a constant conflict in those who care for him (physically or emotionally); they are caught between "I'm so glad you're safe!" and "You idiot, don't ever do that again!" While Snape may not *like* Harry, he's obligated to look out for his welfare, and knows he's important… so he gets the full impact of both those emotions. It's easy to imagine that "I'm glad your alive" leads to an embrace and sparks a kiss (followed by shock from both characters); equally easy to imagine "Don't do that again" leading to punishment fraught with erotic undertones.
Most Snarry has Snape as the "dominant" person, the one who directs the relationship, establishes the ground rules for their interactions and sets limits on what they will & won't do. This seems the most likely according to canon—Snape is older, more experienced, and has extensive practice in both controlling his own emotions and manipulating others. Harry is a teenager; it's easy to assume that lust will override any good sense he's got, and he'll go along with whatever guidelines someone else creates. Still, it's interesting when it goes the other way. Sometimes, Snape is so guilt-ridden or repressed that Harry takes charge, leads him out of his darkness or joins him in it. (These fics are generally set well after the books, after Voldemort is defeated & Harry is an adult.) Snape is an adult; his personality & habits can be assumed to be fairly stable. Harry, in comparison, is young… how he interacts with others (especially people in positions of authority) will change drastically over the next few years. Also, he's being pushed into being A Hero, and Heroes are supposed to be confident & in control.
Speaking of "stories set after the books"—one common premise is "The war is over, the Good Guys won, and Harry returns to Hogwarts to teach Defense Against Dark Arts." And the tension between him & Snape begins either exactly where it left off in the books, with distrust and contempt all around, combined with a grudging tolerance because the other person is respected by trusted friends—or it's assumed as part of the background that they came to know & trust each other during the battles with Voldemort & his minions; sometimes, it's even assumed they've got an established relationship from that time.
I enjoy many of these stories (like many other Snarry fans, I'm not-too-patiently waiting for the next installment of "The Mirror of Maybe"), but I also feel they miss out on one of the best possibilities for tension—the illicit, dubiously-consensual teacher/student connection.
Neither character cares about "rules"… but neither wants to be evicted from Hogwarts, either. The books are utterly lacking in details about wizarding concepts of moral law; we're forced to assume it's similar to Muggle beliefs in the UK, and that any student/teacher pair are likely to be expelled, with the teacher possibly being imprisoned. This means any "relationship" must be clandestine (unless the author takes great liberties with the laws; both The Courting of HP and First Time can be considered AU).
Many of the best fics have them both keenly aware of the illegality of their actions. In some ("If you Are Prepared"), they have Dumbledore's tacit approval; in others ("Bittersweet Potion"), they are aware that anyone who becomes aware of them, is required to send Snape to Azkaban. Many short, PWP fics assume they'll both be caught and punished if they are discovered… clandestine relations during detention, both of them fighting to "act normal" during Potions classes & meals. In some of them, it's obvious to the reader that Snape is misleading Harry, that only Snape would be punished, but he's manipulating Harry into thinking otherwise; these stories are often chan, or border on it.
Some people believe almost all Snarry is chan; after all, if Harry is under 18 (16 in the UK?), it's "kiddie porn" by technicalities. In my mind, chan involves *pre-pubescent* sexual activity; a normal 15-year-old getting laid is not chan, even if he's getting it from a teacher. However, that does lead into psychological abuse issues—there are reasons therapists are forbidden to get involved with their patients. It's called transference: where the "client" (student, patient, parishioner, whatever) is grateful to the counselor, and starts to have friendly or romantic feelings for the person who's been so helpful to him. Many of the fics include this concept, at least unconsciously… Harry realizes that Snape has "always been there for him," and as he grows aware of his own sexuality, he naturally looks to Snape to keep him from getting hurt, and help him find a set of limitations he can live with.
However, Harry's in a unique situation: there aren't many people, perhaps none, who truly perceive him as an equal; his "peers" see him as a celebrity and most of the adults in his life are either trying to save his life, or kill him. Snape's contempt is not pleasant, but it's *honest*, and for a teenage boy caught in a web of power-games and evasions and semi-truths, that can be powerfully attractive.
Some HP/SS stories *are* chan, however, and it often squicks even those readers who like it. There's something compelling about the meeting of manipulation and innocence. (While Harry isn't all about innocence, nor is Snape only manipulative, those are generally the aspects emphasized in chanslash.) We're fascinated—and horrified—to realize that the boy filled with sweetness and honor not only can't defeat experience and lust…he often doesn't want to. He wants to be accepted, be loved, and if not on his chosen terms, he's willing to accept *any* affection as preferable to total neglect.
Almost all good Snarry exploits that tension, the conflict between Harry's need for guidance and Snape's need for respect, and both of their desires for pleasure. In the "happy" stories, Harry's courage and joie de vivre overcome Snape's bitterness and mistrust; in the "unhappy" ones, Harry's impulsiveness and desperation fall prey to Snape's experience and manipulation. In many of them, both are happening at the same time; Snarry WIPs are especially painful because the reader isn't just interested in "what happens next," but "which side of the internal conflict will win out in the end?" In the very best stories, it's left ambiguous: there is no winner, and the conflict is only ended (not resolved) if one character dies.