I don't see the glass as half-full or half-empty. I'd just say "Hey! Who drank out of my cup?" XD
But, being more serious, I'd say it depends on my mood, the time of day, the situation, the stress level, etc. But still, as I said in my first sentence, I'm pretty much neither optimistic or pessimistic. Or maybe I'm a pessimistic optimist or an optimistic pessimist. I don't know.