My biggest pet peeve is when people constantly talk about the past. It bothers me. The worst part is, most of my favorite friends are like this :) It gets to the point where I start to get bored with conversation. Talking about the past and dwelling on it doesn't help. Talking about the future boosts one's creativity and helps develop strategies. Yesterday is in the past, today and tomorrow are the realities.
People seem to feel safer in the past, so they hold onto it, even if it was a terrible past. The future isn't created yet, so many people will talk about what they want, but they won't go far enough to actually start working toward it.