I'm sure the people who research this stuff to earn a living will vigorously disagree, and big pharma will also. But what causes depression and other mental illness has been pretty much all figured out. And we know how to cure it (or at least prevent it)
In hunter gatherer societies (there were still quite a few in existence as recently as the 20th century) the incidence of depression and mental illness is zero. Suicides are zero. Seriously. Let that sink in.
For 2 million years of our existence, humans spent 99% of their time outdoors (except for the time spent sleeping, maybe in a cave). For 2 million years humans did physical labor, every day, all day, such as hunting (which involved lots of running) or foraging for food. No one sat around being sedentary doing soul-crushing bullshit spreadsheets and attending pointless idiotic meetings.
The further you get away from this 2 million years of ingrained evolutionary physiology, higher the rate of mental illness and suicides.
In countries outside of the USA, where big pharma does not own the entire government as they do in the USA, they did some studies and found out, making people exercise had a dramatic reduction in depression. Spending more time in the great outdoors had a dramatic reduction in depression. And believe it or not, fasting (not eating food for a few days) had a dramatic reduction in depression.
The first two most people will grasp, but the 3rd one (fasting) is a bit of a head-scratcher. But not really. For 2 million years people were hunter-gathers. There would've been times of plenty of food and times of scacity. Our bodies are exquisitely adapted and evolved to store food during good times (by converting excess food into body fat) and survive the lean times by using up the stored fat. By not using this mechanism, and just build up body fat store and keep building it and building it and never ever using it (which is what most Americans do), your body cries out in protest. It's a huge imbalance. You are MEANT to burn fat for survival once in a while. You are not meant to stuff your fact 6 times a day, every day, 365 times a year. (The average American eats 6.3 times a day). Fasting is not just something religious fanatics used to do 2000 years ago. It's actually something that every human needs to do to, at least once in a while, because our bodies have 2 million years of evolutionary pressure to do this.