In all probability
Most people believe that if you flip a coin over and over while counting the side it lands on each time, you will find that it lands on heads or tails exactly half of the time. These people could not be more wrong. The mathematics of probability teaches us that this is hardly ever the case, except perhaps at the beginning of this exercise. You will find that the more you flip the coin the further it will delineate from the 50-50 ratio of heads and tails. It is not until you have taken a large sample in the exercise the ratio will again approach (but almost never reach) the 50-50 chance that most people presume will occur naturally.
I know it's just an expression that you can look at a glass of water as half empty or half full, but this is not unlike the coin flipping metaphor. It just doesn't fit into a working model, because it's an oversimplification of a world of infinite variables. Philosophy is too complex to work as a science because it considers some aspects of existence to be more valuable than others. Take the working metaphor of the filled glass: Before a glass is half full/half empty it can be at any state of fullness of emptiness. A glass could be maybe a quarter full and mostly empty, or maybe completely full only to be drank until it is indeed half full.
The glass is just a vessel representing ourselves of course, the water representing a life giving force, and the amount (and predisposition towards empty of full) as a matter of perception. The presumption of course is that if the glass were completely empty it could just be filled again. Naturally, it doesn't really work this way, does it? The glass just can't instantly go from empty to full, rather it is filled gradually until it reaches the desired state of fullness. But in this metaphor, the state of fullness is a matter of perception that can be altered by choice. Empirically we know this not to be true - it is what it is.
We live in a world where people want to be believe that everything is a matter of choice. The perception is such that it is thought anyone can choose to buy a branded medication or the cheap generic (never mind financial barriers). Or perhaps a child could become a chef, a fireman, a stock trader, a musician, or even the president of the united states (never mind an education or the resources needed to obtain one of quality). An addict can choose to have their next fix or become sober (never mind their psychological needs), or a person can read the book or watch the filmed version of the story (same thing, right?), and a depressed person can choose to be happy again. But, of course, it isn't really that simple. Life cannot be changed like the flick of a switch through a simple choice. It is a gradual process of progress before the desired goal can be obtained. What happens when the glass is empty? We might check up on the glass when it is empty and come back with a pitcher of water or be satisfied if the glass completely full, but that vessel is in an in-between state most of the time.
When the glass is mostly empty, at the point where it is unlikely to be even half full again (and very likely to become completely empty) why don't we offer to re-fill the glass? Why do we continue to argue that it will probably be full again, as if it were only a matter of perception? We should we feel grateful that there is at least some water in the glass, but why should we feel guilty to ask for more water? The glass is what it is, but changing that state cannot be achieved by a matter of mere perception. Choice alone is not enough. It also takes resources beyond effort to achieve the desired state.
Unless you have a never ending glass of water, I would suggest picking up the next penny you find heads up. It may be the best choice you make all day. And put it to use, because there is always the chance that nobody else will do it for you. Until we realize that things can only get worse when they are staying the same way, we might not make the effort to try making it better. Make this more than a metaphor - add a variable. Help the process (interfere) in an independent experiment.