Basically life events. It's when one cannot move forward. Something happened or some current situation cannot be resolved. It's psychological. That's why all those antidepressant pills say that they help - in association with therapy. Latest research (a few weeks ago) is that these drugs dumb down your brain. In other words, one probably feels better because the brain isn't so active anymore.

The best solution is brain entrainment through something like HPP recordings. Lloyd Glaubermann does them best.