The Theme of "The Catcher in The Rye" is one of Idealism. Holden is an Idealist, he always thinks of things as they should be not as they are.
Here are a few Examples of Holden being or acting like an Idealist:
He thought that it was so depressing that Mr. Spencer smelled like Cold Medicine.
Holden thought that the little kid running around the streets of New York while following her parents was dumb. Because her parents were not paying attention to her while she was in the street. But he didn't do anything about it.
He thought that telling people all the "David Copperfield" kind of crap was depressing.
Holden feels that he is an Adult, yet he still does many childish things.