Not only that but there will be times, for whatever reason, a nice person will occasionally behave like a jerk. be it because something bad has happened at home or they are just having a bad day.
Always a good idea to consider that the other person isn't having the same day/thoughts/experience as you.
Around 10 years ago I started to approach situations that were dicey by making the assumption that the other person was having a bad day. In addition to helping prevent situations from escalating out of control this has had a calming effect on me.
Empathy is much better than judgment especially in a situation with imperfect information.
http://en.wikipedia.org/wiki/Fundamental_attribution_error
I always try to give the benefit of the doubt. I can think of plenty of times where I only realize after the fact that I was distracted/in a bad mood and unduly acted rude towards someone.
I agree the world is not divided into jerks/nice people. There's a spectrum. Try to stay on the nice side. You can't always avoid jerks but one should not go out of their way to be around them.