What can we learn from Holden's attitude towards Jane, Sunny and Sally on his mental condition?
In The Catcher in the Rye, Holden is lonely (physically and emotionally), and so he calls up Sonny and Sally who will satisfy his need to talk, neck, or have sex. But, he refuses to call up Jane for these reasons. And when Holden learns of her possible physical relationship with Stradlater, he feels betrayed. Holden is in denial about his feelings for Jane; he wants her to remain a child virgin, not a sexually active adult. He therefore feels betrayed by her lack of innocence, and he is paranoid that other girls, like his sister, will soon cross the threshold into the adult world.
Holden cares most for Jane, a girl he used to play checkers with. He loves the fact she's a conservative like him because she used to keep her kings in the back row. He likes her so much that he only innocently kisses her face and not her mouth, which would be a prelude to sex. He also knows a secret about her: her father has abused her. For these reasons, he wants to protect her innocence. But once he learns that she's going out with Stradlater, a self-aggrandising lothario, he runs away from school.
Comparatively, Holden has no relationships with Sonny or Sally. They are phonies compared to Jane. Holden says this about Sally:
I wasn't too crazy about her, but I'd known her for years. I used to think she was quite intelligent, in my stupidity. The reason I did was because she knew quite a lot about the theater and plays and literature and all that stuff. If somebody knows quite a lot about those things, it takes you quite a while to find out whether they're really stupid or not. It took me years to find it out, in old Sally's case. I think I'd have found it out a lot sooner if we hadn't necked so damn much. My big trouble is, I always sort of think whoever I'm necking is a pretty intelligent person. It hasn't got a goddam thing to do with it, but I keep thinking it anyway.
Holden seems willing to neck with Sally or have sex with Sonny simply because they don't mean anything to him. They are simply means to an end (the loss of his innocence). Holden would never neck with Jane, however, because she is quite sensitive and intelligent, and he wouldn't want her to become a phony like other girls.