In these latter chapters of the book Holden carries himself further and further towards his impending breakdown, but I think begins to realize what he is doing to those around him. I found these chapters really quite sad and depressing.
Holden's breakdown reaches its climax in Chapter 25. As the chapter begins, Holden feels surrounded on all sides by ugliness and phoniness?the profanity on the walls, the vulgar Christmas-tree delivery men, the empty pomp of Christmas?and his recent interactions with Phoebe and Mr. Antolini have left him feeling completely lonely and alienated. As he wanders the streets of New York, he looks at children and prays to Allie to keep him from disappearing as the ducks disappeared and as Allie himself disappeared. It's clear that Mr. Antolini was, at least in part, correct: Holden does not feel connected to his environment. He imagines that he is an a mere presence that could vanish at any moment.
Not only does he feel that he can't relate to anybody, but he doesn't know how to deal with adults. As a result, he makes the only decision that makes sense in such a situation: he decides to run away. Unable to deal with the world around him, and realizing that his cynical view of the world is not grounded in reality, he decides to leave.
Phoebe demands to go with Holden, but it is unclear whether she needs him or he needs her. Despite her young age, I would say that she has a clearer perspective on the situation than Holden, so the latter explanation seems more likely. Holden sees the effect his plans have on someone he cares about. A first sign of maturity brewing. He begins to come out of his shell, demonstrating concern for Phoebe and a...