You are asking the wrong question. You have to ask why would you want to be pessimistic at all when you can be optimistic? The answer is control, if you expect things to go bad, and they do, you feel more in control. If you expect things to go bad, and they turn out good, you are still fine. The biggest "hit" to you is expecting things to go well when they don't. Being pessimistic in some ways is a defense mechanism.