At every restaurant, the waiters inundate the glasses so even a slight movement on the table or a small, tentative pause before a sip, could spill the drink. The other day, my mom served my dad's friend a whole glass of orange juice only to discover that he was hyperglycemic. At the movies, you pay $3.99 for a small soda, which inadvertently is left almost full in the theater's empty seats. As different as these situations might be, they're analogous by the fact that the drink -- either water, juice, soda or coffee -- is wasted because quite frankly, no one drinks a full glass. I mean, to substantiate my argument, next time "people-watch" at McDonald's. People keep their cups underneath the vendor, until they are filled with Dr. Pepper, sit down, drink a couple of sips, and throw it away: half way done. Why in the first place would you take too much? Again, I answer my own question: for respect. for dignity.
If a waiter, one day, someplace, poured half a cup of water, he would be looked down upon, shamed, and maybe even fired. Because, filling a glass all the way + serving it = I respect you. It's the reason we, yes me included (hence the we), perpetuate this cycle, this custom, this tradition. As the childhood phrase goes: treat others the way you want to be treated. Treat people with respect (filling a glass all the way), and in return, you'll be respected.
So, ask the world: is the glass half full or half empty? The world will answer half empty, because people are never satisfied with a half cup. I, on the other hand, am waiting for the day someone serves me half a glass of water.