Half Full vs. Half Empty
A long time ago, my wife and I were having a discussion, and she chided me for being a "glass half empty" person. This immediately erupted into a "marital discussion", since I had always viewed myself as a "glass half full" person - an optimist.
This debate raged for years without any resolution.
Then, one day, she had showed me something, and I began to comment on how it could have been done differently and thus improved. She said "There you go again - half empty." In frustration, I exclaimed "I"m not saying the glass is half empty... I'm saying that it's only half full. What I'm saying is - YOU COULD FILL IT UP!!"
Suddenly, I realized the problem. In viewing the world as "only half full" I see all of the possibilities for improvement. But, this comes across as negative to those who worked so hard to fill it up halfway. As a result, to them, I become a "half empty" person.
Is this important? Probably not, if you're one of the people that are content with "half full" or "half empty" and you see yourself as others around you see yourself. But, maybe it is if, like me, you're just asking the world to "fill it up the rest of the way".
 - For those not familiar with the reference, some people look at a glass that is filled halfway with water and say it is "half full". Others refere to the same glass as "half empty". This metaphor has been used to equate "half full" people with an optimistic view on life, and "half empty" people as those with a pessimistic outlook on life.