I have dealt with huge, soul crushing depression. In my case it was absolutely a spiritual/lifestyle problem. Specifically, my core values involve personal growth, and contribution to society, and I felt incapable of either. I had a good job, a girlfriend, good relationships with family, etc, but none of that mattered. The rational I had was that my in the big scheme of things I didn't matter - I could just as well not exist, and outside of a couple of people absolutely nothing would change.

I feel like the massively connected nature of modern society is directly responsible for flourishing depression. Odds are, you are redundant; if most people disappeared suddenly, someone else could probably pick up the pieces in fairly short order. Additionally, being hyper-connected means that whatever you're good at, it is probably easy to find examples of people who are much better at it than you, which can be be somewhat marginalizing. Not feeling necessary or exceptional in any way is a pretty sure fire recipe for angst.

The nasty thing about depression is that it is a self-reinforcing cycle. When you start to get depressed, your ability to take measures to address the root cause of the depression is impacted. The result is a downward spiral that results in self obviation, through suicide, self destructive behavior or substance abuse.