Here we go...
Depression is currently thought to be a combination of low serotonin levels in the brain, and some kind of event that brings on the the disease.
For me, it was a combination of growing up in an alcoholic household, moving every two to three years, physical shame, and being an awkward child/teenager.
The first coping mechanisms I learned revolved around anger and shutting myself off from the people around me. A lot of this stems from reaching out and not getting what I needed. As I got older, the more the anger and dark thoughts ruled my days. I didn't start coming out of this until my late twenties.
I taught myself to find other ways to live with depression, and one of the greatest joys for me was training other people wherever I was working. If I could help someone else gain/hone the tools needed to be successful, it helped me feel better. No anger, no hateful thoughts. Trying to be a better person.
I am currently working backwards through my life with a psychologist, untangling knots and tearing down walls. I can honestly say I know what fear is, now. This is hard.
And if you have, or know someone that has, depression, then you'll understand why I can't breathe right now.
What lies behind you and what lies in front of you, pales in comparison to what lies inside of you.
-Ralph Waldo Emerson