IMO most cases of depression are caused by life events. That opinion is based on seeing a number of people close to me suffer from it, and recover. Generally it is not entirely obvious to the person what is causing the depression, but looking closely at their life usually pins down a cause. Generally it is something along the lines of working in a job that you don't like, being in a relationship that isn't working, excessive stress/burnout, or not having a purpose in life, or some combination of these factors.