Just saw this provocative title and article on BBC. There's got to be an evolutionary basis for everything. One of the purposes of happiness has got to be approach. Happy people approach others and share their happiness. Sad people withdraw. At least one purpose of depression is to slow down, stop what you're doing, and reassess.

But often these systems go out of whack, and you can drive yourself crazy wondering whether there's a reason for you to be depressed right now. In fact, that's a distortion mentioned in cognitive-behavioral therapy, where the patient is constantly finding reasons and circumstances that are making her depressed.

Aristotle said something like: "It is easy to fly into a passion--anybody can do that--but to be angry with the right person to the right extent and at the right time with the right object and in the right way--that is not easy, and it is not everyone who can do it."

Likewise, there's times when being depressed is probably very useful. The most obvious one that comes to mind is when you're in an abusive relationship. Being depressed may force you to reflect and reassess yourself, giving you the necessary gumption to get out.