I'm an engineer. The glass is not half-full or half-empty, the glass is twice as big as it needs to be.

As Zargabaath and Fate pointed out, it depends on if the glass is being filled or emptied. If you're filling it up, it's half-full -- it's halfway to where it will be. If you're emptying it, it's half-empty -- again, halfway to where it will be when you're done with it.

While RagnaToad pointed out that the physical issue may not be the object of discussion here, it could still apply. If somebody expects bad things to happen to them (the glass is emptying), they'd look at it as half-empty, as more bad things (emptying) are on the way. If somebody expects more good things to happen to them, they'll look on the "bright side", that the half-full glass will be full when it's done, and that things are going to get better for them.

I usually go with a "half-empty" attitude. It's not that I'm a pessimist. I'm a realist. But reality sucks.