If you're an optimist, you're happy that your glass is still half full. If you're a pessimist, you can't help but notice that 50% of your glass is now empty — that is, it contains nothing.
Empty comes from an Old English word meaning "at leisure," or in other words, "unoccupied." Over time, the sense of that meaning changed from "not at work" to "devoid of contents." By the tenth century, any container that could be filled or full could also be emptied or empty. As the word continued to evolve, it developed a more figurative sense. If you feel sad and lost, you might say that you feel so empty. If you want to go to the movies and your parents say, "No, but maybe tomorrow," you probably consider that an empty promise, one that will never be fulfilled.