After skimming some reviews on Goodreads, it seems like they're all some expression or combination of the following two sentiments: this book is annoying and this book is relatable. People who really like the book probably really relate to it, and people who hate the book (a large group) probably don't. I fall into the former group of people. Of course, there are people who relate to it and still hate it, and people who don't relate to it and still like it. But these are exceptions. Anyways, here are some thoughts.
Holden's main problem is that he is both very judgmental and very empathetic. Most of his behaviors can be explained by these two qualities. Everything depresses him because he is too empathetic. For instance, the new ice skates:
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 questions - and here I was getting the ax again. (67)
Here, Holden puts himself in his mother's position. He imagines the effort that went into buying the gift, and the subsequent disappointment his mom would feel upon learning he'd been expelled. It's an oversimplification. To him, the gift and his behavior are connected. Given his bad behavior, he views himself as undeserving of the gift. Gifts make him sad because he doesn't properly receive them. They're a waste of effort on the part of the giver, and an unnecessary obligation for the receiver.
Similarly, Ackley depresses Holden because he over empathizes. He imagines what Ackley must feel like and turns those feelings into his own.
Holden thinks everyone is a phony. However, he's also a phony. He constantly lies: he tells servers that he's old enough to drink, he gives the elevator man a fake identity, etc. In his words, he loves "shooting the crap." I think he does this because he's too judgmental. When interacting, he is hyper aware of what the other person must be thinking or feeling. He then assumes that other people must be judging him equally as much. This makes him put up a facade; he is almost never sincere.
Anyways, I don't want to write too much about this book. I could, but I won't. Heck, one last comment. I liked the repetition of the "that kills me" line. I could never really tell if he meant it cracks him up, or if it depresses him. Maybe it's always both.
Here's a cool article about why empathy is bad