I don't think we're one or the other; I think everything is a mixture; we're much more complex than we understand. There's situational (being sad because no one would be your friend as a child, that's a social "trauma" too not just a chemical reaction!) and there's chemical brought on by how we live and eat and/or were "born" with deficiencies, etc. There are a zillion ways, most of them unknown how our chemistry can impact how we feel. For all one knows, one can have sensitivities to meds one takes to help with a problem! I have adult asthma, not related to any allergies but probably sparked by a too-high/too-much doses of the "wrong" antibiotics when my appendix burst in 2003 and I had a wandering infection for 5 months. But, I was born with my lungs being a "hot spot" for me from genetics. But all sorts of things have cascaded since 2003 and there's no way to untangle/fix some of them.

There's no such thing as a 100% depression either; it is a continuum and that means that some percentage is "okay" and I think focusing on that percentage and areas and working to make them grow as much as possible is always helpful. I think of it a little like I do cancer where they have patients imagine the good cells as "sharks" attacking the bad, cancerous cells, etc.? Champion healthy parts of yourself?