The theme of the novel is loneliness, modern existential loneliness exacertabed by the horrors of the recent war and the looming threat of a nuclear World War III. Holden Caulfield is especially vulnerable because he obviously has a near-genius I.Q. This explains why he is able, at only sixteen, to write a full-length novel full of sensitive character descriptions and original observations about all sorts of things. (He is, of course, the creation of a much older man, J. D. Salinger; but Salinger manages to create a convincing illusion that we are actually reading the words of an intelligent, lonely, confused adlescent boy.)But his high I.Q. sets him off from other people. They can't understand what he is talking about. His encounter with the young prostitute is a good example of the distance between Holden and the average person.
Holden only vaguely understands that his own parents do not really want him. That is why they sent him off to boarding school. Those elitist private schools are full of unwanted children who are strangers in their own homes when they return on holidays.
Holden is his own worst enemy. He is looking for companionship, but he is so discriminating that he rejects nearly everyone he meets for one reason or another. He considers a lot of people "phony." This is undoubtedly true of a lot of people. They do not want the kind of intimacy that Holden is seeking. That kind of intimacy is rare in the world. Most people wear masks to interact with others. This is especially true among people who live in big cities--and New York is the biggest city in America. Holden is naive to think that he is going to get much sincere contact with a bunch of strangers during his short visit to a relatively strange city where everybody is struggling for survival in a Darwinian-Malthusian environment.
Two prominent themes are individual perception and the response to tragedy.
Central to the story is the tragic death of 14 children. Different characters respond differently to the tragedy, for example, Billy begins drinking. The entire town is shaped by the tragic event. For the most part, the tragedy unifies and uplifts the town.
The theme of individual perception is tied to that of tragedy: different people respond differently to the same event. Certainly, every dialogue or character commentary on the even displays a unique perspective, however, the scene in which Billy and Mitchell meet in the garage and discuss the same event, each having a very different perception of it.
The theme is the loneliness and self isolation of mental illness, PTSD. Holden complains incessantly about being loney, and yet he self-sabotages all his relationships with people other than his little sister, whom he idealizes and idolizes.
He picks arguments and fights with his roommates. Whether outwardly or inwardly through internal dialogue, Holden denigrates and offends classmates and teachers and even a woman on the train, for whom he concocts an outrageous lie about her son, one of his classmates. He provokes arguments with a former teacher and an upperclassman and with a girl that he has a date with. He even succeeds in offending cab drivers. Desperate for company, he pays a prostitute and then sabotages that transaction, insulting her pimp by calling him a "moron," provoking him to punch him.
No one can help Holden in his downward spiral of mental crisis because the adults aren't qualified to comprehend his situation, and the only person he levels with to ask for help is someone too young to understand or to do much good even if she did realize he has a mental condition, his little sister, which guarantees that he won't be helped until he collapses.