Well, quoting Alejandro Jodorowsky: " A glass isn't half full or half empty. It contains a certain amount of water" (please bear in mind that this is a translation made by me -spanish to english-, so it's pretty literal).
So half empty or half full are really human conceptions, basically how you see is a projection of yourself. Also, I invite everyone to have their own interpretation of that quote =] .
And that concludes my philosophy babbling... cheers