I know some people who play both sides, because if their team wins, well great, that's what they wanted all along. But if they play bad/lose, then they can say "well, I told you so", so they get a satisfaction of being "right". It's like hedging a bet.
It's also that pessimism sounds like someone's trying to help you to understand something you're ignorant of, whereas optimism comes across like Pollyanna.