After the 70s, doctors and psychologists began to realize this isn't just because your life circumstances are crap or because something traumatic has happened. What they identified that as being is a chemical imbalance in your brain. And it's not just that your brain isn't producing enough of these happy chemicals,. It's that your brain is having difficulty transmitting them, processing them, and using them for the correct reasons. That's what the most common type of antidepressants aim to fix.