What's very intriguing is how common resilience to depression is despite exposure to one or even multiple risk factors. I think it's something as high as 90% of the non-depressed population will experience at least one risk factor and then there are big chunks with 2-3 and you get some people who were abused as a child, have a stressful job and drink regularly who are not depressed as well.
This is why the most popular hypothesis about depression have a biological, psychological and social component. It's probably that people are prone to depression due to their genes and sometimes environment can influence its severity, sometimes not at all.
I had a partner who had a loving family, no childhood trauma (I mean, you had to scrape the barrel to find it - which her psychologist did *sigh*), didn't drink or smoke, ate healthy, exercised regularly, (edit: she was very conventionally attractive as well), lived in a warm sunny country, had a well paying and not too stressful job, stars were aligned and she was absolutely destroyed by anxiety and had mild depression