Do you see this right here? Dean making sure Sam has his books and then saying he doesn’t need books, himself, because he won’t be around long enough anyway. That pattern-the one where Dean believes Sam’s life is worth more than his own–that’s being established right here. Dean obviously knows that books are necessary to be successful at school because he makes sure Sam has his. He can see that Sam is successful at school when Sam’s jumping around too, so Dean knows the jumping around thing isn’t what’s going to stop him from doing well. But Dean’s already learned from John that he’s a good soldier and nothing more. He’s already given up any hope for anything different for himself. Following what John’s been telling him his whole life–look out for Sammy. Never: look out for yourself.