OK - Clinical/reactive depression - usually caused by situations that the person is unable to alter. Can be left over from childhood events or recent trauma.
Organic depression - caused when the chemicals in the brain don't work as intended. I suffer with both. I don't use the word 'suffer' lightly. I wrote a tome about my journey with depression, if I can find it I'll bump it.
I HATE with a vengeance the 'pre-disposition' theory. When I was initially ill in the 1980s I asked for investigations to find out why I was depressed. My paternal history is a grand-father who drank and gambled and a Dad with manic depression. Maternal granny had what would be described now as post natal depression and was hospitalised for 4 months after her youngest child was born. So no one was interested in finding out causation. It is what it is in my case. Treated with anti-depressant medication which I fought and then accepted. I have to take something for Life. Even then events can over-take me, if so I up my dose of ADs for 5-6 days which usually helps. Dad had various medications over the years as well as ECT and deep sleep treatment to rest the brain.
I found talking therapy really useful: discuss, decide, ditch. As well as appropriate medication. If it works it helps. I would suggest to take meds for 6-8 months and review, not short term as many GPs will prescribe, i.e. 4-6 months. It can take a while for my brain to realise that it is feeling well again

Does it matter? If someone is depressed it is better to treat the person rather than wondering why, whether it be causation, trauma, hormones ......