It's gonna be different for everyone. I've actually heard that the chemical imbalance theory has been challenged, with some people saying that we should look more towards inefficient coping strategies and basic physical health as contributing factors to depression. (Generally backed by Big Pharma having all their fingers in the doctor's pies, so to speak, which IS true.)

I personally think it's a combination of both worlds; how your brain is shaped throughout your childhood development and from your hereditary genes, and also by traps that we can fall in psychologically and physically. That's why cognitive behavioral therapy can provide relief to some people, who might be battling with negative thought trap spirals, and not do a thing for others who's problems might be more chemically based.

The point is everyone is different. And trying to pinpoint one specific cause for depression, which is a unique illness in and of itself, is like scooping up a handful of sand and calling it a "beach".