Is the glass half empty of half full? If you choose the former, you're a pessimist, if you choose the latter, you're abnormally optimistic. So which is it to be?
How about this for an explanation?:
If the glass is being filled up, it is half full, if the amount is diminishing, it becomes half empty.
What if the level is neither increasing nor decreasing? you ask. This generally can be said to depend on the last action on the glass - if it was last added to, it is half full, if some of the liquid was last taken away, it can be declared half empty.
It's that simple.