The glass should be half empty

I was wondering, why do they call people who see as the galss being
half full as optimistic? In my view it should be exactly opposite. The
people who see the glass as half full are actually missing out on the
potential of the glass to accomodate the whole double of the water it
already has. Any one who is having a glass half filled is just going
to sit there and not do anything about it cause, hey! he _has_ a glass
half _full_. So basically if we have more and more people who see the
glass as half full, the economy will just go on degrading, inflation
will drop down, and bascially everything will come to a grounding
halt. And besides economics such an attitude will hamper social growth
as well. Half a freedom is never a freedom but then again with half
full people may be no one will notice.
So basically i think seeing a glass as half full is being passimist.
Seeing that the glass is half empty, seeing the potential to grow,
acknowledging that there exists a problem, in the end casuing the
problem to be solved and not just sit there it is true optimism.
But then again i have many other weird opinions too....