Work and school had me indoors 60+ hours a week, so lots of stress and no Vitamin D.
I had little to no time for cooking, so I ate junk processed fast food for most meals.
All of these things left me so drained that I stopped exercising and didn't have the energy to go to social events. I stopped hanging out with my best friends.
What little time and energy I had left was spent laying on the couch indoors on my phone obsessing over the news (which of course is overtly negative and divisive) and arguing politics on social media.
Anti-depressant drugs provided marginal relief from a complete breakdown but it wasn't until I started "working to live", getting outdoors, eating healthy, exercising frequently, meditating (when needed), quit watching the news and deleted my social media accounts that my mental health returned.
There are a handful of simple things that human beings require for adequate mental health or "chemical balance", and yet there are massive societal pressures and trillions of dollars being spent sabotaging those basic human needs. Unfortunately it's much easier to pop a pill than to confront such difficulties.