This is actually a very complex question that lies at the very heart of the character of Holden. He is clearly a very troubled and disturbed young man, as demonstrated by his complete apathy towards life and his inability to stay in any school. Obviously the death of his brother is something that is probably behind the behaviour that Holden demonstrates, but in addition to that his apathy and his inability to make any definite change in his life is also, at least in part, due to the way he feels that life is against him. Note what he says after Spencer gives him a lecture about "playing by the rules" in life:
Game, my ass. Some game. If you get on the side where all the hot-shots are, then it’s a game, all right—I’ll admit that. But if you get on the other side, where there aren’t any hot-shots, then what’s a game about it? Nothing. No game.
Holden already identifies himself as being with the "other side," the side where everybody else is who is not popular, talented or gifted. His particular attitude to life, which is characterised by an inability to make any decisions and to change anything concretely about his life, is therefore down to his sense of being victimised, of not "fitting in" to the world and that he is disadvantaged in some way. Why bother trying to make something of your life if everything is stacked against you?