I think that biological and environmental factors play a part in the origins of depression. Some people are born more prone to having depression, however they may not encounter certain situations in their life that will activate it. Maybe depression seems to be more common now is because it is more openly talked about, or that people live in such distracting and stress inducing environments?
Furthermore, I was raised as a Roman Catholic. Having faith or spirituality gives some people a reason for living or being of service to others. Praying is a form of meditation as well. It is very strange too, that something troubling will be on my mind, and I could listen to a sermon and it would calm me down. So pretty much, I think having faith does reduce depression. Maybe I read a study about it somewhere. Spirituality could be an environmental factor as well...for example, my parents could have been atheist and raised me with no beliefs-it could have turned out I would feel even more lost than I do now? I am just rambling now. But I believe in more than one reason for depression.