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Then I went over and laid down on Ely's bed. Boy, did I feel rotten. I felt so damn lonesome. (7.29)
Arguably the most powerful cause of Holden's depression is his feeling isolated from the rest of the world. Even though he's judged Ackley to be pimply and disgusting (and painted a portrait of him as a social outcast), he still reaches out to the guy when he feels lonely. Much of the tension in Holden's character arises from the conflict between his hating the world and his wanting to connect with the world.
Everybody was asleep or out or home for the weekend, and it was very, very quiet and depressing in the corridor. […] All of a sudden, I decided what I'd really do, I'd get the hell out of Pencey – right that same night and all. I mean not wait till Wednesday or anything. I just didn't want to hang around any more. It made me too sad and lonesome. […] Besides, I sort of needed a little vacation. My nerves were shot. They really were. (7.58)
Again, it's the solitude of the empty corridor that Holden finds depressing. His statement that his "nerves [are] shot" reminds us that he is, in fact, headed for some sort of breakdown.
One thing about packing depressed me a little. I had to pack these brand-new ice skates my mother had practically just sent me a couple of days before. That depressed me. I could see my mother going in Spaulding's and asking the salesman a million dopy [sic] questions – and here I was getting the ax again. It made me feel pretty sad. She bought me the wrong kind of skates – I wanted racing skates and she bought hockey – but it made me sad anyway. Almost every time somebody gives me a present, it ends up making me sad (7.60)
Again, what's sad about this? In this particular case, Holden's sad that his mother went to all this trouble to buy him skates, and now he's let her down by "getting the ax" at yet another school. When he says that he finds any present from anyone to be depressing, it seems as though he feels he doesn't ever deserve gifts.