People who see happiness as just the material aspects of their life are never going to be able to maintain that happiness sustainably. I think feeling sad is a natural reaction to situations where we should feel emotional or upset, we should feel pain. But much like other emotions, sadness is temporary and it should fade with time. It becomes a problem when we find ourselves in these prolonged periods of sadness that are really difficult to get out of. The difference between sadness and a depressive episode or depression is that sadness can often be treated.