When I was younger, I used to be pretty pessimistic and skeptical. I was distrustful of other people and I was very distrustful of the world at large. During this time, I was (not surprisingly) unhappy. I don't recall the exact point that I made the decision, but one day, I decided that I wasn't going to live like that anymore. I decided that I was going to treat people like I wanted to be treated. I decided that I was going to assume everyone was a good person at first and let them prove me wrong.
Later I read "How to Win Friends and Influence People" by Dale Carnegie. In the first chapter, he talked about how everyone things they are the hero of the story. One anecdote he told really stuck with me. It was the story of a gangster in the 1920's who wrote a manifesto while he was held up in a house surrounded by the police and before he died. This gangster was a horrible person who had murdered people, but in his manifesto, he only talked about how he had be wronged, how he'd only done what he had to to protect himself. We all self-justify our actions, and by treating people like they are bad only makes them resent you.
Basically what I'm saying is I always treat people like they are good people trying to do good things, because that's how everyone sees themselves. When you treat everyone like good people trying to do good, everyone treats you better too. It's a self-reinforcing cycle. Maybe just give it a try the next time some one is rude or wrong. Treat them like they are good people trying to do good things that made a mistake.