A theme in The Catcher in the Rye is the alienation and loneliness of youth.
Holden suffers from the same condition most teenagers do: he feels that no one understands him or appreciates him. Holden has taken his feelings to extreme levels, because he is an exceptionally sensitive and introspective young man. His angst often reveals itself in anger and depression.
I can't stand that stuff. It drives me crazy. It makes me so depressed I go crazy. (ch 2)
Such a comment, about the phoniness of his prep school, is common from Holden as he tries to find sincerity in the adult world. While small things depress him, little things interest him too. When he hears the little kid singing about the catcher in the rye, he feels better because the kid is just “singing for the hell of it” (ch 16).
Holden wants to connect with people, but does not know how. Even with Phoebe, he is not sure what to do. He worries that if he gets close to people, they will disappoint him. His need for connection and lack of ability to pursue it create an increased level of depression and anxiety for him.
Loss of innocence is another theme. Children most obviously represent innocence and purity in the book, and they are virtually the only people that Holden appears to like and connect with. His sister Phoebe and dead younger brother Allie are the children that he's most fond of, but his longing for a connection (or re-connection) to the innocent and happy world of childhood also comes up in other ways. For instance, there is the little girl he meets in the park, Phoebe's schoolmate, whom he praises for being 'polite', and he remarks that kids usually are. He also feels happy when among the Christmas shoppers with 'a million little kids' around. There is also the kid who walks in front of him along the street, singing, who cheers him up:
It made me feel better. It made me feel not so depressed any more.
For Holden, children represent the world of joy and innocence to which he longs desperately to return. For him, the innocence is now lost, but he wants to retrieve it. This longing gives rise to the book's title, as he pictures himself out in the fields with a lot of 'little kids' near the edge of a 'crazy cliff', where his only job is to save the kids from falling over the cliff. This is symbolic of his wistful, and futile desire to preserve the innocence of children, to stop them falling into the world of adulthood with all its attendant problems and complexities.