How do you be an optimist?

Think opportunity cost!

"Man, I broke my toy dog. But that's ok, I'll end up spending less time and battery on it and have more space for my toy kitten!"

How do you be a pessimist?

Think opportunity cost again!

"Ok it's the school holidays, but that means I'll have less time with my friends, less time in the nice library, likely to get bored, more likely to overspend etc. Oh bother."

So what separates the two? Well, the pessimist thinks more about what he loses, while the optimist thinks more about what he gains. Not so much of glass half full or half empty.