Here's the difference between an optimist and a pessimist:
The scenario is that someone laughs at how big your shirt is (like someone did to me last week) and says, "Wow, that shirt is huge! That's not yours, is it?"
If you're a pessimist, you'll think they're saying it's because they didn't realize how fat you are (and then you'll cry).
If you're an optimist, you'll think they're saying it's because you look smaller than you think you do (because you're in denial).
And if you’re a realist you'll know that they're saying it because you wear a XXXL that looks like a parachute with sleeves.