There are many sorts of depression. Some are mostly situational (eg loss or failure), some are mostly chemical (eg Bipolar Depression), some are mostly psychological (eg a big blow to a fragile ego, or self-contempt either deserved or undeserved), etc, etc. Many or most, it seems to me, are mixtures.
In my view, most people with a depression need some sort of psychological intervention or help, and many benefit from medicine unless the medicine completely sweeps things under the rug that ought to be addressed. Depression is a non-specific phenomenon, like pain. It is, in fact, mental pain. Sometimes, mental agony.
Interesting article about Ketamine ("Special K" as it is sold in nightclubs). If it's one more tool in the toolkit, great.