WHY IS DEPRESSION SO WIDESPREAD?
"Depression is a systemic condition involving, as we know, cognitive, emotional, motivalional, behavioral and bodily aspects of the psychosomatic (body and mind) operating system.
To understand why it is that depression arises with the breakdown of the human life operating system, we need to understand the precise nature of this mechanism that effectively runs our lives. In other words, what sort of a mechanism, when it breaks down, manifests as a dramatic swing from happiness to depression, from hope to hopelessness, from a sense of self-control to a feeling of helplessness?
Research is now showing that at the human level, the mody-mind regime operates not on the basis of life preservation but life-satisfaction.
And when that natural "happiness" mechanism breaks down, depression is inevitably the result."
From "What Depression Really Is" - Ch 3 of Depression: What It Is and How To Fix It