Sadness, oddly enough, can sometimes even have a richness to it. When people are crying, for example, they feel a cathartic element, as if they're connecting to something very deep within them. There's even something that may (oddly enough) feel good about a good cry, about feeling sad in certain situations. There's nothing rich about depression. Sadness can be a motivating factor for a lot of people. If they're sad, they're unhappy about a particular situation, it may inspire them to want to get up and do something about it. Depression is usually the exact opposite and that it saps a person's will. It saps a person's motivation to feel that there is nothing that can be done about a particular situation. People who feel sad (if you ask them) will usually admit that there is a chance that a year from now, regardless of what's going on, they may feel a little bit better. But in depression, hopelessness is one of the core symptoms where people will feel that there is no chance of things ever improving and the way they feel right now is how they're going to feel forever.