Spoilers: Spoilers for a scene in OotP.
Disclaimer: Snape belongs to J.K. Rowling. This is only one of the reasons I'm jealous.
Author's Note: Dipping my toes back into the Harry Potter fandom for a quick slice of - what else? - Snapefic.
He wonders, once again, if he miscalculated. The idea bears a sharp and a bitter taste, but Severus Snape is a man well accustomed to bitterness.
The Pensieve was a dangerous balancing act. He doesn't give a damn for Harry Potter's pity either way, and whatever span of humiliating memories flashed between them, he doesn't care. But some things hold more dangerous truths than just the residue of a blunt and brutal life.
He should have remembered that the standards to which Harry Potter is held a good boy are rather looser than they should be... and that curiosity has probably killed more schoolboys than it ever did cats. Cats, at least, have a certain sense of slick self-preservation.
Who would dive headlong into a fully immersive set of memories without knowing when their owner might return to catch him at it? Harry Potter would. He has to remember sometimes that merely surviving all manner of dangerous situations is no guarantee of acquiring good sense... and the boy has a damnable habit of poking about in corners.
And drawing conclusions: some wrong, and all hasty. He wonders what Potter thinks he would have to hide away in his Pensieve? Embarrassments and wicked secrets, no doubt. Teenage boys always see the world in such laughably simplified terms.
He wonders how much Potter saw, and how much he will make of it. Denial is a powerful force - but unfortunately, the boy has not sufficient cowardice to hide in it. He'll remember what he saw, and the only hope is to cloud the circumstances surrounding it. For while he doesn't give a damn if Harry Potter thinks him broken, scarred or pitiable, his empathy and understanding are a different matter.
His reaction was an instinctive move, and perhaps indeed a foolish one. The boy needs this tuition, true enough, and to yank it away in an attempt at causing senseless injury may cause a worse disaster than it fixes... but somebody else will have to tend to that, for Snape has a more important role to play.
Everyone in the wizarding world no doubt has their own opinion of what a boy like Potter needs: support; sympathy; companionship; protection. Severus Snape knows better.
Harry Potter needs an enemy.
Voldemort should be enemy enough for one boy to handle, but he's the wrong sort of adversary; a raging force on the distant horizon, as outsized and impersonal as thunderstorm. He may have wormed his way into every aspect of the boy's life and childhood since the year it began, but he'll never belong to Harry. Potter may oppose him, but he'll never be able to truly make an enemy of him, any more than he could make an enemy of death.
Voldemort may be his ultimate foe, but it's not an ultimate foe that Potter needs. He needs an adversary, someone to pit his strengths against... because strengths, one way or another, must be pitted - and where no enemy is found, one must be made.
Malfoy fancies himself an enemy to Harry, but he's twice the fool his father is. He's a child, still playing at death and destruction with no more sense of their reality than an infant waving a tree-branch for a wand. Snape had known that Harry would outgrow him long before the Potter boy began to realise that he had. If Malfoy was the only enemy Harry had, he'd soon learn how quickly the hunter becomes the hunted.
But that will never happen, because Potter still has Snape. Not a spiteful weakling like Malfoy, but the face of Authority - immutable, unchallengeable. Snape has power over Potter, and he wields it with a steely, iron hand, because if he didn't, the boy would be lost. He doesn't realise he has power of his own, but in a world where all were weaker, he would use it.
Potter will never understand the root of Snape's hostility, but that's exactly as it should be. It has to be irrational, for that's the only thing that could work on a boy like Harry Potter. The Pensieve is a worry, not for the raking over of old humiliations, but because it raises the risk of sympathy and understanding - and that would destroy everything he's worked so hard to build here.
Terminating the Occlumency lessons was a rash decision, and perhaps a bad one - but he'd needed a disproportionate reaction, and that was the only card he'd had high enough to play. If there's debris to be cleared up in the aftermath, Albus will have to take care of it, because he's taken the only path that was an option. Potter needs to have Snape as his enemy.
As long as his enemies are stronger than he is, he won't turn into James.