So maybe low serotonin levels are caused by depression? My naive understanding is that these amines play a critical role in our emotional response, social interactions and other complex psyche, so as a consequence, depressed person suffers decline in these functions, as well as a whole higher nervous system suffers regression. I always had a feeling that SSRIs are treating consequence, not the cause. Maybe I'm too naive and my understanding is very poor, I just architect systems in the end, so I'm looking at this problem like at a bug in the very root of an unimaginably complex system, while treating it by boosting one or another amine is looking like the most ugly hotfix.