Some people are primarily pessimists, always seeing life through a dark glass. Others are optimists, always expecting the best outcome from every turn in the road. A little pessimism is good. It prevents the unwary from falling victim to scams and predators. But too much pessimism makes for a very negative life.
Optimists tend to be cheerful and calm. After all, they're expecting good things...right?
As with all things, too much of a good thing can be destructive or dangerous. When my son jumped off the six foot wall in front of our house, his unwavering faith that he would be all right didn't prevent him from breaking most of the bones in his feet. That's where a little bit of healthy pessimism would have come in handy.
With all things in life balance is the key.