I have a completely unscientific, subjective theory that some significant portion of depression in the population as a whole is caused by the economic circumstances that some people live in. Things like being trapped in a dead end job to keep healthcare benefits in the USA, living in a never ending treadmill of debt.
IOW: Are you ready for me to diagnose your totally normal reaction to our shitty society as mental illness so I can get you addicted to mind altering drugs for the rest of your life? https://imgur.com/Jb1mJyx Or as the Unabomber said, "Imagine a society that subjects people to conditions that make them terribly unhappy then gives them the drugs to take away their unhappiness. Science fiction? It is already happening…
Not to mention, depression takes several forms, and not all of them are related to unhappiness. A common form is related to anxiety, with people experiencing a constant feeling that something awful is about to happen, sometimes getting terrible panic attacks. Given that many of these people live in objectively and subjectively extremely safe environments, much safer than at any other time in history, it's very hard to take seriously any environmental theory of this kind.
Edit: corrected one bad auto-correct typo ("problem", not "purulente") .