Purely anecdotally, the first time that I suffered from depression, I was in my teens. This lasted for about five years and was defiantly situational–I.e. once I got out of the situation, I slowly recovered. There were really good reasons for me to be depressed–hormones, being the outcast at my high school, my father dying.
But I find now that the depression can come on really suddenly–sometimes with no reason at all. For instance, I could be driving home from a perfectly normal day, and suddenly be overwhelmed with incredible hopelessness. Or I will wake up and want to sob. Just sudden, unexplainable, incredible sadness.
There are times that I know there is a reason to be depressed–something bad happened, or I think of something disturbing–but I recover more quickly from these incidents than the other kind, which can sometimes last days.
Mood disorders do run through my father’s side of the family, and Dad was bipolar, but I appear to be the only person in my generation to suffer from depression–yay me.
I saw–not too long ago, (but no cite) that people who suffer from untreated depression tend to be more likely to suffer from bouts of depression later in life–as if the brain gets trained somehow to be depressed, and I’m pretty sure that’s what happened to me–a genetic susceptibility combined with a history of it.
So I am a combo platter of both depression with a reason and reasonless depression.
Hope that helps with the OP.