If you are a true pessimist you would always expect the worst. So even when good happens you would be unable to enjoy it, more often than not, because you would expect bad to come of it. If you are a true optimist, you at least aspire to good happening and, when it doesn't, you hope for the best to come about. You can rarely be half and half in the deal and always feel great.
But you shouldn't expect things, anyway. :o