[cpncrunch]

It's difficult to determine whether or not depression is caused by life situation, unless there is something obvious like a recent bereavement.

It seems likely that both external stressors and internal thought processes can both be the cause of depression (and in many cases, a combination). CBT addresses the internal thought processes, but doesn't do anything to address external factors. My wife underwent CBT and took antidepressants, but they didn't really cure the depression. Quitting her crappy job fixed it for good.