I've actually answered this question before, but it's one I love so I'll share my opinion.
It would entirely depend on what the glass is half full or empty of.
Example 1 - Half filled with gold, woohoo, I have a half glass of gold.
Example 2 - Half filled with poop. Well, at least it's only a half a glass of s#%t.
Does that make me an optimist? No.
Does that make me a pessimist? No.
It makes me someone who sees a half a glass of whatever it may be filled with.