I've often been accused of being a pessimist. I'd rather like to think of myself as a realist.
I do, like most people, however, dislike being around grumpy people. These people have also been termed pessimists. They seem to always expect the worst in people---and many times, they eventually are correct.
I have developed my philosophy after many years of observation and testing: expect the best, be prepared for the worst. You've probably heard this before, the concept is certainly not new. But why should we expect the best? Through my observations, I've learned that people tend to eventually get what they expect. This is a very strange result, but in my experience and tests, it seems to have some statistical relevance.
My theory is that there are a couple of reasons for this. One involves other people, and the other involves yourself. People are really good at perceiving expectation of others. Generally, people tend to follow the norms present around them---thus, if you want people to behave a certain way, simply be surprised when they do not. Another reason expecting the best seems to work is that it changes your own attitude. No longer are you frightened to try new things, no longer are you bitter, no longer are you stuck.