The empty half of a glass

I wonder why we tend more and more to see the empty half of a glass instead of the full one?

Is it probably, because the full part reminds us of something that has already been achieved and doesn't motivate us anymore and what we don't have is more interesting and exciting that we tend to see it easier?

I also have the stange ideea that we are very atracted of different kind of shapes, that we consider perfect(in fact not even the line or a round isn't perfect:)) and we don't have space for the unknown in our so complicated life-style.

In fact it is quite difficult to focuse only on the part that you have and try to forget about the part that is empty, but i have the feeling that the empty part is always going to be empty somehow,because you always find things that you don't have yet...So?

So been too atracted of the empty half of the glass you might loose the essence and probably in the end you'll have a totally empty glass in front of your eyes. Is it worth?

P.S. : I have seen lately a quite interesting,funny and sincere interview, which was somehow the beginning for this post: http://www.youtube.com/watch?v=8On0nBGeQok