The most prominent theme in this novel relates to Holden's inability to let go of the past and move into the future. We can call this a fixation on innocence.
Holden romanticizes and clings to memories of his dead brother Allie. There is no point at which Holden learns to accept Allie's death as being real or permanent. He speaks with Allie when he gets depressed and stressed, as he does in the New York hotel room. Holden also recalls a time when he felt he was disappearing and he called on Allie to save him.
The death of his brother haunts Holden. A number of details from the narrative suggest that Holden's emotional growth and maturity are stunted by his younger brother's death.
Holden's relationship to his sister also demonstrates his strong connection to childhood and his inability to let it go. He is fixated on Phoebe, on how great she is, and he repeatedly states that she is the only person who understands him. She is his only real friend.
Holden's saving grace is his little sister Phoebe, whose clarity and compassion—if somewhat idealized—give him the strength to carry on.
The two characters that Holden idealizes, his younger siblings, are exemplars of innocence and childhood. The many people he derides are sophisticated, jaded, mature, or simply grown up.
There are many things and many people that spark distaste in Holden. The majority of them represent a loss of innocence in one way or another.
Holden's run-in with the prostitute and her pimp and his drink date with Carl Luce are both episodes where we see Holden invovled in a failed and troubled attempt to become more sophisticated and leave his childhood behind. Each episode ends in depression for Holden and becomes a near disaster.
It is innocence that Holden craves and which bouys him when he is with his sister. Yet, it is the inevitable loss of innocence that plagues him. He struggles to keep a distance between himself and any future loss of innocence by clinging to the past and to figures of innocence. Thus he ends up back in the park with Phoebe.
The park evokes his own fond memories of childhood, before his brother Allie's death, and seeing Phoebe circling around in this natural setting seems to bring him a sense of permanency and wholeness.
I would say that the main theme of Salinger's novel The Catcher in the Rye is loneliness. The fact that most of the story takes place in America's biggest city makes Holden's loneliness only more obvious by contrast. He seems to be a loner at heart, an introvert, an outsider, an onlooker. All his actions, including his pathetic adventure with the young prostitute, are attempts to break out of his isolation. He is a loner partly because he is exceptionally intelligent, not unlike some of the characters in his stories about the Glass family, published in the New Yorker and then in book form in Franny and Zooey, Nine Stories, and others.. He probably has a genius IQ but is not aware of it because he is too young to realize how much he differs from most of the people he encounters. The insights in his first-person narrative are sufficient to show that he is an exceptiionally intelligent young man, even though he keeps flunking out of schools. Being an introvert with a genius IQ makes him a misfit. People cannot understand him--and he cannot understand them. No doubt Salinger, who lived in seclusion for much of his life, was writing about himself.