The glass contains half the amount of its capacity.

Me: ...I'm always pissed at something
jimbo: yea, I know this. You hate the world
me: only select parts of it. You could say my world is peppered with hate
jimbo: heavily peppered?
me: regularly peppered but with fresh, ground pepper from a grinder. Big chunks
jimbo: lol big chucks of hatred.
me: I'm not that bad, am I?
jimbo: lol, not always. But I think you might view life through tainted lenses.
me: yea I'm a pessimist
jimbo: but that's ok it makes you funny
me: I'm such a pessimist I hate the glass half empty analogy.. The glass being half full or empty is relative to how it got to that state.. If you fill a glass halfway, its half full. If you pour water out, its half empty.
jimbo: this is true
me: so when someone asks me if the is glass half empty, I say it depends. I cant make an assumption without further information. So people that just say it is half empty or half full aren't pessimists or optimists, they're just guessers.
jimbo: good point. I never looked at it that way
me: no one does.

Furthermore, if you really wanted to analize it, you could say that the glass being half empty is the result of you just taking a drink and replenishing yourself. since you've just quenched your thirst, you are in a positive state and therefor being optimistic about the glass. I think the whole "glass half empty" analogy is just too full of holes to be accepted so widely.

http://en.wikipedia.org/wiki/Pessimism