Way back in elementary school we had a substitute who insisted that the pessimist was the one who saw the glass as half full. Her reasoning was that she didn't like to drink milk so, to her, a glass half full was bad and half empty was good. We thought she was dumb.
True optimists are usually unhappy. They expect the best, which rarely happens, and they are always let down. True optimists rarely ever consider the worst case scenario and are poorly prepared. Meaning, if the worst happens, it is really, really bad.
Pessimists expect the worst, prepare properly for it, and are always pleasantly surprised when things are better than they thought. And, if the worst did happen, it isn't very bad, because the pessimist was prepared for it. True pessimists tend to be happy.
People tend to confuse optimism with being happy. It isn't whether the glass is half full or half empty, it is what you can do with that glass that matters.