I thought it was fairly obvious that Holden changed because of the death of his brother and it made him a more bitter, internalized person. That's why he smashed the window, and mostly ran away from everything and did everything that led up to him being institutionalized (where the book begins). My last post was kind of tongue in cheek. Obviously I get the more subtle meanings of it.
Also, +1 for what Joker said about the loss of childhood innocence and his frustrations with that. That's why he gets upset at reading the scrawled "Fuck You"'s written on the walls, with the thought of Phoebe having to read it. That's something that would pull her into the adult world and it's not what he wants for her, himself, or anyone.
Damn, now I'm feeling like reading it again for the 945th time.
