It pisses me off so much when people use their car horns to signify anger. Acceptable reasons, in my opinion, to use a horn include:
1) Someone is in danger and you want to warn them.
2) You need someone to move and they are not aware of this.
Even #2 is a soft call, but I honestly don't mind so much if I'm, say, at a light and the person behind me honks to get me to move forward a foot so they can slip by to make a right turn. That's fine. Or if I zone out and the light changes... I don't love being honked at, but I can understand it and feel it serves a purpose. I've done it before and I try to honk very gently so they know I'm not angry.
But when someone does something you don't like and you honk just to show them you're pissed off when you're not in any danger of hitting them? That really stinks. And I feel like it makes people less likely to respect horns when they really need to be respected.