Meh.

I dip out. It's a noted characteristic of mine since college; I leave parties when I want to leave parties, I don't tell people, especially if I think people are going to protest or fake-protest my leaving. When I'm done, I'm done. It may be a party or a bar but the point of the matter is when I no longer want to be somewhere I leave, regardless.

At this point in life I expect my close friends to know this about me and accept it. Sometimes the dramatic ones forget. Recently I had someone accuse my (now ex) boyfriend of being "rude" because we left a bar where he and I left a bar where we were supposed to meet said friend...after being there for four hours and waiting for my friend for two hours. It's funny how what really was my prediliction for leaving got shoved onto the boyfriend because the friend didn't want to connect it to a known tendency of mine.

Essentially, people should be smart enough to know their friends' tendencies, but sometimes they don't or don't want to acknowledge it.

It sounds, however, like maybe you were frustrated by your friend venting for an extended period of time. Maybe your friend's flaw is that he likes to vent. Maybe handling that flaw might mean changing the subject instead of pointing out how much he is venting?