Pissed when people ignore hellos
I don't know if this is common everywhere, but I often find that in my town people don't return friendly greetings. If they simply didn't hear or something, it would be understandable.
I'm talking about when you're passing someone in the Wal-Mart parking lot or at the entrance of a convenience store and say "Hello" or "Good evening" while smiling and the person just ignores you. They don't look at you. You know they heard you. They don't do anything to acknowledge that you did anything. They just pretend it never happened and keep right on going. It pisses me off when people are like that.
If I had to sum it up in a couple short sentences as an explanation aimed at these people, it would be: Look, I don't know who you are but I'm trying to be friendly to you and pretend I give a sh*t that you exist. The least you could do is acknowledge me.