\\it's not that we don't care,
we just know that the fight
ain't fair
so we keep on waiting
waiting on the world to
change.\\
Sam grew up believing in all the things that go bump in the night, like most little boys. Unlike most boys, however, he never grew out of that belief. Things grabbing him from under his bed, being strangled half to death by an enraged poltergeist, the mind-numbing sound of the first time he saw his father shoot down a werewolf, the backlash of the rifle as he himself shot a dryad through the heart years later-- all these cemented his belief like nothing else could. He couldn't really blame his dad for not telling him it wasn't real, when they dealt with the things on a daily basis, when John Winchester was quickly becoming a name feared throughout the supernatural world.
He didn't want to believe it, but he did.
When he was thirteen he decided he wasn't, just wasn't, going to believe it any more. His history teacher had said that faith was the most powerful motivator of all, and Sam knew, he knew, that if he ignored the monsters they'd leave him alone. Other kids weren't attacked by the monster under the bed on a weekly basis, after all. Disbelief worked for them, it had to work for him too.
He never forgot the look in his father's eyes that night, when Dean had to forcibly drag him out from under the hotel bed they were sharing because he hadn't put up a line of salt.
"What were you thinking, Sammy!" And the accusations were coming from both sides, from all around him, and all he could say was I'm sorry, I'm sorry, I'm sorry. I'll never do it again.
He kept that promise until he couldn't handle it anymore. Then he ran for Stanford, away from his family, away from the monsters, the poltergeists, from all things supernatural.
He broke that promise hoping that he'd never need it again-- no more hunting, no more fear-- but the evil followed him. He broke that promise until the day Dean appeared back in his life, and then he never broke it again.
Because waiting for the world to change was one thing, and helping it along was another thing entirely.