I tend to follow the idea that everything starts out empty. When your class first opened at say 12 midnight in November for the Spring semester, it was at first empty. Then over time it filled up. Starting out fast, then to a trickle. So it would be half empty.
However, this idea still never stops me from saying a glass that I lay eyes on is half full :) I just think we are taught to be happy and to think positively. It's been told to us by our parents and relatives and teachers and friends. We just get used to it.