An optimist sees the best in the world, while a pessimist sees only the worst. An optimist finds the positive in the negative, and a pessimist can only find the negative in the positive.
For example, as a duck hunter, I was in the market for a new bird dog a few years ago. My search finally ended when I found a dog that could not only retrieve but could actually walk on water! Shocked by my find, I was both skeptical that the dog would be able to keep this up after I purchased it (you know, maybe the salesman was somehow playing a cruel trick on me), and also that any of my friends would ever believe me.