The novel works through the psychological lens, as its entire plot relies on a coming-of-age narrator in psychiatric treatment. Salinger also infuses the work with Freudian psychology (see the reference link below for more on this).
Holden opens his narrative (the novel's first paragraph) through a psychological lens:
If you really want to hear about it, the first thing you'll probably want to know is where I was born, and what my lousy childhood was like, and how my parents were occupied and all before they had me ...
His diction reveals his psychic tension: does the psychiatrist/therapist really want to hear about it? The context suggests that the it is the ambiguous (yet obvious) source of Holden's mental and physical exhaustion; he experienced an existential crisis. After Allie's death, Holden loses all sense of self and his place in the world; Holden believes Allie deserved to live in his place, as Allie embodied for Holden all he values in living.
Holden loses his hold on the world as he perceives it should be—that is, innocent. He suffers from arrested development. Holden's attempt to deviate from his innocence code results in failing out of another school, failing in social relationships, and placing himself in dangerous and violent situations (for example, his fight with Stradlater and Maurice's attack). When Holden describes the violence to his intended audience (the psychiatrist), Holden reveals his innermost psyche; he only knows he is alive when he experiences physical pain or innocent joy (as with Phoebe). There is no space for living between the extremes. Daily reality exists between the extremes. Holden can no longer function in a "phony" world.
According to Freudian psychology, Holden lacks a developed ego (identity as an "I" with real world reasoning); therefore, his id (immaturity, fears, childish reasoning, and unconscious desires) unconsciously controls his real world functioning. Holden's id finally exhausts his physical, emotional, and psychological health—hence, he narrates the story from a psychiatric care facility:
Besides, I'm not going to tell you my whole goddamn autobiography or anything. I'll just tell you about this madman stuff that happened to me around last Christmas just before I got pretty run-down and had to come out here and take it easy.
Salinger structures the novel to be read as a psychological study; the reader has access to Holden's private therapy session—the psychiatrist/therapist is the intended audience, not the reader. Salinger challenges the reader to "want to hear about it" through a psychological lens.
Holden refers to his "madman stuff," suggesting his id recognition. He needed to deal with reality in an appropriate manner, but in order to do so, he needed a stable ego. Without a stable ego, his id drained his energy. Now, he needs to stop running away. He needs to rest so that he can appropriately process Allie's death and the ensuing trauma from his loss.
The novel's final chapter closes the therapy session:
That's all I'm going to talk about ... A lot of people, especially this one psychoanalyst guy they have here, keeps asking me if I'm going to apply myself when I go back to school next September ... I think I am, but how do I know? I swear it's a stupid question.
Holden's unwillingness to accept and answer the world's "stupid" questions illustrate the novel's broader psychological lens: humans have a need to talk and a need for someone to listen, really listen. Holden's talk therapy permits him to feel alive, to think about living in the present moment not the past or the future. If the reader accepts the novel as one therapy session (which it is), and really wants to hear about it (Holden's crisis), then the novel is highly effective through the psychological lens.