If one becomes inexplicably depressed , with no known current cognitive/emotional issues, either internal or external, that, would imo, probably point to a persistent endogenous or organic cause. I'm a 40 year+ on/off chronic depressive, and can experience it spontaneously, even when my life seems to be going well and basically happy in myself. Being acutely depressed over one-off life issues, such as job loss and bereavement are responses to known events... and natural. The ability to distinguish between endogenous and reactive states is likely down to assessing behaviour patterns and life events timeline. There is everything in between, of course.