I think the "low serotonin = depression" thing is widespread because the old view was that because serotonin boosters helped depression, depression must be caused by low serotonin. Some psychologists may have never moved beyond stating that simplistic explanation, I guess maybe because it works for the psychologist's purposes. At least depression isn't then viewed as a moral failure or that MDD patients families insist they should just will themselves out of severe depression. You may not be able to will yourself out of some mental illness any more than you can will yourself to grow 6 more inches. Not that cognitive behavioral therapy and attempting to consciously avoid rumination isn't helpful.
The new view is more along the lines of serotonergic signaling, and everything downstream of that (including functional, morphological and genetic changes) may oppose depressive pathology.
Hey, sorry for not expanding on that original link. I don't necessarily agree with the author that low serotonin causes depression, in fact I'm not necessarily sure that's what he is saying. But is it correct to say that those with low serotonin (whether or not it's a cause or symptom, I'm not sure it matters) present with those symptoms? And therefore SSRIs can be extremely helpful.
Here is another article from him discussing the other main neurotransmitters targeted by medication:
http://www.drjoecarver.com/clients/49355/File/Chemical Imbalance.html
I accept the article is old, the somewhat simplistic explanation and that research has come on leaps and bounds. Having said that, please correct me if I'm wrong, the drugs of choice used in treatment remain largely the same?
My symptoms for example match the "low serotonin" diagnosis perfectly and if research has shown those presenting with those symptoms benefit from SSRI's then is right to discount him?