I find this discussion interesting.
As I haven't done any study into depression, I don't know very much, but I do have my own experiences to draw on. I have suffered from depression for over half my life and am realising that for me at least, it is always going to "be there", but that I can manage it.
In my case, events can trigger me into a "bout" of depression when my emotions close down and the whole world is grey. I can recognise this in myself whilst or even before it happens now and can make sure I do something about it.
I take anti-depressants. Maybe there is some placebo effect, but even if there is then I feel that it is worth it. Without the anti-depressants I find that I start to spiral down into a grey person, who does not enjoy life.
I have tried various forms of counselling. I am very good at opening up at first and getting everything off my chest but then feel there is little else to be done or said. There is no specific trauma that has happened to me and I think it is more a case of how I deal with things.
I haven't tried CBT, which I think could possibly help me, but I have a book on it which I will get round to reading one of these days .
I am pretty sure that my mum suffers from depression, although it is not something we have ever discussed. I don't think she is being treated for it, although again I may be wrong. I also believe that her father had it as well.
So although I can see that there may be no proof that there is a genetic link, I think that I possibly am pre-disposed towards some amount of depression.
I control it, rather than it controlling me. To me that makes the difference.