Depressed people often have trouble performing everyday activities, they can’t concentrate on their work, they tend to socially isolate themselves, they are lethargic, and they often lose the ability to take pleasure from such activities such as eating and sex. Some can plunge into severe, lengthy, and even life-threatening bouts of depression.
So what could be so useful about depression? Depressed people often think intensely about their problems. These thoughts are called ruminations; they are persistent and depressed people have difficulty thinking about anything else. Numerous studies have also shown that this thinking style is often highly analytical. They dwell on a complex problem, breaking it down into smaller components, which are considered one at a time.
This analytical style of thought, of course, can be very productive. Each component is not as difficult, so the problem becomes more tractable. Indeed, when you are faced with a difficult problem, such as a math problem, feeling depressed is often a useful response that may help you analyze and solve it...Analysis requires a lot of uninterrupted thought, and depression coordinates many changes in the body to help people analyze their problems without getting distracted...
Sometimes people are reluctant to disclose the reason for their depression because it is embarrassing or sensitive, they find it painful, they believe they must soldier on and ignore them, or they have difficulty putting their complex internal struggles into words.
But depression is nature’s way of telling you that you’ve got complex social problems that the mind is intent on solving. Therapies should try to encourage depressive rumination rather than try to stop it, and they should focus on trying to help people solve the problems that trigger their bouts of depression. (There are several effective therapies that focus on just this.) It is also essential, in instances where there is resistance to discussing ruminations, that the therapist try to identify and dismantle those barriers.
Andrews, P. W., & Thomson Jr, J. A. (2009). The bright side of being blue: Depression as an adaptation for analyzing complex problems. Psychological Review. 116 (3), 620.