This is the dilemma that was addressed to me in university. Does depression decrease Serotonin or does a lack of Serotonin cause depression? The jury has been out for a long time. Yes, Serotonin treatment alleviates the symptoms. I don’t think the medical model relies on drugs alone for depression. It also recommends counseling. The Seretonan model is also 50 years old. (I just found that out.)
“Overall, this evidence suggests that impairing serotonin function can cause clinical depression in some circumstances, but is neither necessary nor sufficient.”
I mentioned before, when I had clients with symptoms of depression, we would hold sessions by going for walks. I had very good success with this. (I worked on a university campus and there were plenty of places to walk.) This would not always be an option and, some depressions are justified. Someone is experiencing depression in the form of homesickness. Getting out, walking, making friends, getting to know your neighborhood and interacting with others can be part of the cure. If your dog died, … not so much. There is a time and place for everything. Any treatment has to match the situation.