Weegy: All of us will experience sadness at some point in our lives. Depression, however, is a physical illness with many more symptoms than an unhappy mood. Depression often has no logical reason. [ People with clinical depression are not able to just "snap out of it". A person who suffers from a major depressive disorder must either have a depressed mood or a loss of interest or pleasure in daily activities consistently for at least a two-week period. This mood must represent a change from the person's normal mood and impair his functioning in his daily life. ] (More)