As per my understanding it is a chemical imbalance in the brain blah blah blah.
Sometimes I wonder if most cases of depression are merely a psychological effect and not entirely physical. Going even further with that when I was younger and didn't leave the house for weeks on end (playing video games, what else?) my mom took me to a psychologist and a psychiatrist. I went to like 10 sessions total and I was diagnosed with depression. I was given some meds to take and that was that. I had no choice in the matter, I was depressed. That made me mad that a label was put on me, I didn't feel sad in any way. I just never went out during that summer because it was time to play some games. My friends always came over and played with me, I don't see what the big deal was.
But I ended up not taking any of the meds because it would interfere with my Ritalin I took for ADHD.