Holden misses Maurice. Let's just start with that. Maurice is an angry pimp who prostitutes out a young girl, steals five dollars, and punches Holden in the stomach. And then Holden misses him. What is going on?
Maurice reminds us that Holden is lonely in every way. The whole reason Holden agrees to the prostitute thing in the first place is that he's "depressed" and has no one to talk to. Since he's still incredibly isolated at the end of the novel, it makes sense that he thinks he "misses" all the people he met over the course of the novel – even Maurice. This brings us back to Holden's basic conflict: the world is full of "phonies and bastards," but he needs to connect with them anyway.
We were also rather interested in the punch-in-the-stomach scene and subsequent bullet-in-the-gut imaginings. Maurice gives us some insight into Holden's self-destructive tendencies. Holden knows Maurice is bigger than him, tougher, stronger, and probably not going to back down from a fight. Still, Holden persists in insulting him until he gets socked – hard – in the stomach. It's almost like Holden wants to get punched, to feel something, anything.
This isn't the only instance of Holden intentionally making things worse for himself. You could argue that he purposefully gets kicked out of Pencey, drinks to dangerous levels, and wanders around the freezing park at night because he's acting out a wish to self-destruct. Yes, we got all that from Maurice. Crazy, isn't it?