Why do some consider despair a disease?

A medical cause: Despair is an awful thing to feel. We know there are a number of medical problems that can cause someone to feel that way even though there have been no events that would lead to it. Some examples include diabetes, anemia, and major depression. We don't use the old adage "pull yourself up by your bootstraps" for someone's diabetes. Why should we use it for a chemical problem in the brain?