I was taught by my driver's ed teacher, many years ago, that the horn is there for a reason. (He would reach over and honk it when my 15-year-old self was too shy to do so.)
---I honk to prevent an accident (e.g., somebody pulling out in front of me)
---I honk (lightly) if somebody doesn't realize the light has turned green
I didn't used to do the latter, but I sat through far too many green lights where the driver was dinking around, didn't realize it was green until 2 seconds before it turned red, giving them just enough time to get through while the rest of us had to wait for the next light. In that sort of situation, I don't blare my horn, but I will quickly beep it, and only if I've been waiting for at least 5-10 seconds and it's clear the driver isn't paying attention (e.g., trying to get something out of the back seat). I don't honk it because I'm pissed off, I just want them to look up and notice.
I did that once several years ago and the woman in front me promptly flipped me off and (from what I could tell from behind) started yelling at me. I must say, this was, um, before my e-hell days, and I should probably have served quite a bit of time in e-hell for what I did next. I was both annoyed and amused by her over-the-top reaction, so when I ended up in the lane next to her, about 20 seconds later, I beeped my horn and with a big grin on my face waved the same finger back at her. She pretty much exploded and looked as though she wanted to kill me.
Nowadays, there's NO way I would do that! To start with, it's horribly rude, and secondly, people like that are liable to hunt you down and claim road rage as a defense!
However, don't assume that a car horn means that somebody is pissed off at you. It's simply a method of communicating with other drivers.
Now, if you'll excuse me, I'm going to go do penance for my past e-hell sins.