Consider taking a third, "Realism," position in which the glass is delightfully observed as being both half full and half empty (i.e. half full of atmosphere) simultaneously. With this mindset, one can begin to leverage influence to modify the situation effectively.
Perhaps half a glass is plenty to satisfy, perhaps the empty half creates a space to contemplate artistically. Importantly, what we do with our observations is the starting point for taking action to positively changing (or accepting the present state of) our self, community and world. Once action has been taken, it's time again to evaluate the situation to see if a difference was made.
At any given moment, an opportunity to fill the half glass may present itself....and when that moment occurs, you just may want to think thrice if a full glass will actually lead to goodness, or a spill waiting to happen.