I think most people doubt that depression is caused by a single biological mechanism and is most likely a holistic/dynamic problem. I think that's why some doubt the value of pursueing such reductionist hypothesis of depression.
I don't think that depression is down to a single biological mechanism, and there is a big difference between depression due to environment and chronic depression. But saying "holistic/dynamic" is not an explanation, but is more like waving your hands around and saying "woooo" a lot.
