I met a Swedish nurse who worked in a ward where they treated young people who'd had mental breakdowns. She said the most common cause was a tough life situation. In the short term they would prescribe drugs, but ultimately their treatment was focused on changing that persons life to remove the negativity which spurned the breakdown. Whether it be encouraging them to get a new job or move out of their current living/ relationship situation.

OTOH, plenty of people go through tough life situations without suffering mental breakdowns.

I think it's more complicated than a simple either-or proposition.

I would be more happy to accept the idea that depression was usually a result of a chemical imbalance in your brain if this was able to be reliably diagnosed by measuring the balance of chemicals in your brain, rather than the current method of divining the supposed chemical imbalance by filling in a short questionnaire.