You need look no further than the way Holden characterises himself as an angry young man, denouncing everybody around him with the label of "phony" and acting in a way that is counter to the kind of behaviour that is expected of him. Consider what he tells us about his reasons for getting thrown out of his current school in Chapter Two:
One of the biggest reasons I left Elkton Hills was because I was surrounded by phonies. That's all. They were coming in the goddam window. for instance, they had this headmaster, Mr. Haas, that was the phoniest bastard I ever met in my life. Ten times worse than old Thurmer. On Sundays, for instance, old Haas went around shaking hands with everybody's parents when they drove up to school. He'd be charming as hell and all. Except if some boy had little old funny-looking parents. You should've seen the way he did with my roommate's parents... I can't stand that stuff. It drive me crazy. It makes me so depressed I go crazy. I hated that goddam Elkton Hills.
One of the central reasons therefore why Holden is such an outsider and chooses to isolate himself from the world is his lack of ability to tolerate the "phoniness" that he sees around him. As his rather extreme reaction to Mr. Haas in this quote shows, "phoniness" is something that drives him "crazy" and makes him "depressed." Of course, the perceptive reader also realises that Holden is clearly struggling with issues of unresolved grief for his brother, which helps focus his rage.