One of the panels I was on was one on Order of the Phoenix. And I totally hogged the panel. I mean, I stepped on other people and shouted things out, and when I didn't, I whispered to the woman sitting next to me. I was really, really bad. There's no excuse.
Except. I kept getting all these *ideas*. And, just like I was a precept hog in college, I had to keep shouting these out.
And only one stuck with me. Maybe because I didn't get a chance to blurt it out.
We were talking about Snape. And this came to me. Columbine.
Think about it. Snape was a classic geek/nerd - the unattractive, greasy, possibly less-than-hygenic kid who is all about studying and D&D or Star Trek or computers or, in his case, potions and Dark Arts. And he's with the unpopular crowd if he's with one at all.
And then we get James, the prince of their year. Handsome, talented, intelligent, athletic, and he hangs around with Sirius who is even handsomer and brighter (top of the class without studying) and from an aristocratic family, to boot. And Remus may be quiet, but he's also bright and popular. And he's a prefect, too. And anyone else had better be like Peter - sycophantish. These *are* the popular crowd, at least in their own minds.
And how do they get their kicks? By teasing the unpopular kid, the nerd. Now, this goes on in every high school. It's only worse at Hogwarts because of magic.
While I don't think it's *just* because of James and Sirius - that's far too simplistic, and I can believe he was a nasty kid - in fact, we know he was. And prejudiced and full of pride in his own pure blood. Anyway, while I doubt it was the only reason, I'm sure it was a factor in his choice to follow Voldemort.
Why he decided to go over to Dumbledore, I don't know. Unless. Unless Dumbledore was fighting to keep him, gaining his trust and respect in anyway he could, during his entire school career. And Snape came back when he saw what Voldemort was - that even there, he was probably despised as a kid or lower class or something.