He's always wished he could be more like the others.
Not out loud, of course; he'd never admit something like that. He supposes that's part of the problem, really: he's so closed, has spent so long being told that emotions aren't to be shown, that he keeps everything inside. Course, he's not brilliant at hiding his emotions – or perhaps they just know him too well – but he's very good at avoiding the subject, at not saying things. And sometimes, all that internalisation gets to be a bit too much. Sometimes, he can't keep his everything locked up inside himself, sometimes things spill over, sometimes he ends up doing something stupid, something reckless, something impulsive. Something like telling Snape about the knot on the Whomping Willow. Something that makes him hate himself, because what sort of fucking idiot would do that? He hates that he does things without thinking, that he lets his fucking feelings get the better of him, rather than just doing what James would do, and talking about it.
Course, he'd never admit that either. He's always teasing James about being such a bloody girl for wanting to talk everything through. To contradict that would be hypocritical.
But it would be so much easier, if he could just talk. Preferably at the time rather than later, when the boys have cornered him after the event, when Peter's angry and James is worried and Remus isn't really sure whether to be upset or disappointed or terrified that he'll do the same again. He doesn't really have a choice about talking then, because the alternative is them hating him, and even just thinking about that is a little too much.
They understand, of course, though he half-hopes they won't. He doesn't really understand it himself, doesn't get why he sometimes feels so out of control of his own actions, and he hates his stupid fucking impulsiveness. He sort of wants them to hate it, too.
He's also a little surprised. James, he expects to understand, because James is like that. James gets him, was worried about him rather than angry, and has such black-and-white views on things that he couldn't hate him, not when he's firmly placed in the "white" side. Remus, too, he expects will forgive him relatively quickly, because that's just the sort of person Remus is. Peter, though, is supposed to be the perpetually cheerful one, the one who's never had an irrational feeling in his life, the least likely person to understand his bloody fucking stupid actions, his inability to just think before acting.
Yet, Peter understands, too, and that makes him think. Maybe he's underestimated Peter. Maybe Peter is like him, hides things, but is just a little better at hiding the emotions that go with them. Maybe Peter doesn't tell them everything.