As a primary care physician who encounters and treats depression weekly (it's one of the most common maladies affecting people in general) I've observed all levels of severity (from the mildest of "blahs" to the most severe suicidal ideation) and have come to the controversial conclusion that the true underlying cause of depression is the same in everyone. You can find the reasons for my believing this by following the link below. I hope you find it helpful.
Happiness in this World » The True Cause Of Depression
Alex