As someone who is hypersensitive to chemicals and fragrances of all kind, I can say without any doubt that in my case depression is caused by environmental toxicity.

I can become suicidally depressed with the flip of a switch, if I'm exposed to the right toxin. In my case it is mold and certain fragrances or perfumes.

And I have no history of depression, and no childhood trauma of any kind. This all happened after severe mold exposure and development of chronic ME + chemical hypersensitivity.

From my point of view I posit to argue that scientists are clueless of the real causes of depression. From my point of view, and repeated own experience and experience of others who suffer from MCS, it is crystal clear that toxicity is a major trigger for depression.

If you are depressed, and don't know why: consider your environment. Are you being exposed to mold or strong chemicals, building materials, furniture VOCs (Volatile Organic Compounds)? You should also consider your diet as being one potential source of toxicity.

Modern healthcare and psychiatry has almost completely disregarded environmental toxicity as potential for disease. I think there is an enormous gap in knowledge here, that is yet to be filled.

Bad can be just as well defined as a lack of good, much like depression itself. Consider that it may be an exercise in futility to try to discern the "true cause" of your emotional state, and that it may be more worthwhile to begin implementing solutions than to attempt to further deconstruct your problem. Human psychology is much like software save for one very important distinction: you cannot read the code. As such, trial and error remains the best strategy.