I can't speak for others, but I think a lot of what he says is funny. There's little to no thought put into any of it - it's' just automatic gainsay of whatever someone else is saying, often devolving into biting epithets like 'poop head', motivated by the belief that you don't lose the argument as long as you can reply with something - anything - no matter how irrelevant or scatological it may be. There's entertainment in that.
I don't mind when he singles me out and insults me, but I expect it to have at least a little comic value. Otherwise, it's just annoying.