The symptoms of depression are know to occur from a wide variety of physiological problems. Wikipedia lists
many of them.
Anyways, that drugs mainly effects only one of the neurotransmitters I listed. I take bupropion, an NRI. It effects a different neurotransmitter, but is mildly effective at keeping my mood from dipping. I'll repeat myself and say that attributing depression to a single neurotransmitter seems too simplistic.