It always bothered me that supposedly when you say a glass is half full you're optimistic and when half empty you're pessimistic. The reason why it was bothersome eluded me until I noticed that when you truly talk about a container with a liquid you sometimes say it is half full and other times half empty. It would be severly strange if one switches their outlook on life that quickly and often.
Therefor it has nothing to do with ones outlook on life or their emotional state. This comes forth from the idea there is one single moment possible for this statement. The truth however is that the glass and its content has a history which is crucial to determine what its half-state is in the now. If a glass was previously full and is now emptied by half, it is half empty. If a glass was previously empty and it is filled to one-half, it is now half full.
Of course this can create a problem when the observer isn't aware of the previous state of a glass. But even then most people will make a judgement based on contextual information to deduce its historical state. For instance; if one is thirsty you will say your glass is half empty to emphasize it was previously full. Or if one is not thirsty you will say the glass is already half full, to emphasize you want the glass to return to its empty state. Another example is when you put gas in your car you fill it up until you are half full. But while driving you notice the gauge in the middle and you remark it is half empty.
This is of course a literal approach to a metaphor. But it shows that the metaphor can only be applied to the situation, not visa versa. So next time someone tells you something is half empty/full, remember that you now know their perception of the history of that container.