I always thought that pessimists are people who are too lazy to even find something great about their life and this world. On the other hand, optimists, for me at least, are those who cheerish even the smallest things in their life and because of that attitude, can smile and say "life is good".
Personally, I stay cheerful by reevaluating my day - what have I eaten, what have I drank, where was I? Sadly, some do not have answers do these questions.