I doubt many pessimists see the good of many situations.
Even if things aren't too bad, they figure they can get a LOT worse.
Now an optimist will always be thinking of the good things that WILL happen, regardless of how bad it is.
Pessimist: cup is half-empty, it's gonna be empty soon.
Optimist: cup is still half full, that means I still have half of it left!
I don't agree pessimists will be happy if things seem OK - they don't believe it will last. They remain convinced that things are gonna get worse: it doesn't make them happy when things get worse, just more convinced they are right to be pessimistic.
Optimists on the other hand always find something to be cheerful and hopeful about. If an optimist and a pessimist get thrown into a cage with hungry lions, pessimist will say, "We're gonna get mauled to death!"

; optimist will say, "I always wanted to pat a lion, now's my chance!"
IMO, anyway.
Cheers
Mick