I'm a very calm and measured dude, but when people don't give the hand wave after I do something nice for you on the road, they're dead to me. It's just one of those things. I usually disagree with most drivers when they have that panicked, "alright YOU FUCKING ASSHOLE" just for drifting a little or something minor (we all make little mistakes on the road). But if I let you pass on a narrow street by pulling to the side and you don't wave to me -- fuck you, dude. Fuck you. You obviously saw me make moves for you, and I'm resting my hand in the wave position on the wheel, so you see that, too. It's a sign of sociopathy to me.
After a while people don't even bother taking the steps you do to show some empathy, because they've been screwed over like that one too many times. Sad world :(