What does seeing the glass half empty mean?

The phrase, seeing the glass as half empty, means that a person is taking a pessimistic view, preferring to see what he or she doesn't have rather than what he or she has. If you have, let us say, a 10 ounce glass which contains 5 ounces of grape juice, you can be happy that you have half a glass of grape juice to drink, or you can be unhappy that you do not have the full 10 ounces of grape juice to drink. You can see the glass as half full, which is a good thing, or you can see the glass as half empty, which is a bad thing, although in either case it is the same glass with the same 5 ounces of grape juice in it.