Everything, more like what doesn't. I am in it right now. Cannot stand it and every time I try to get out of it something happens or someone does something that sucks me back down into it. Feeling like wallowing in it for awhile and see how that goes for *sighs* ignore all that.
It depends what you are talking about exactly. In general, depression can be caused by life events either acute or longer term. But for the average person, their will be periods of mild depression and even some major depression
As for the biochemical reasons serotonin, norepinephrine, and dopamine all play key roles in depression. And one or more of these chemicals might be low. And during a very terrible time those chemicals go down but for the people with Recurrent Major Depressive Disorder those chemicals are naturally low.