Long term depression is also very hard on the people around the depressed individual, which adds to the social isolation and a cycle that maintains (or worsens)

Long term depression is also very hard on the people around the depressed individual, which adds to the social isolation and a cycle that maintains (or worsens) the depression state. States of mental depression almost seem like a possession or a second personality, where the depression state fights to continue itself and subverts other parts of the brain to maintain or deepen the depressive state. Since it is so hard for normal people to give support over the length of depression (which seems hopeless and it doesn't help that the sufferer projects hopelessness), it simply wears out family and friends. It may also be an inherent defense mechanism of "normal" people to compartmentalize depression. Certainly there is enough things in the world that would depress anyone. Hey, you're going to die. Nature is brutal. Life is unfair and unforgiving. Everyone is subjected to a tsunami deluge of advertising trying to make you feel empty and fill it with buying things. I think people are deep down afraid of depression being contagious in some way, because who knows what people's breaking point is.