Depression is a terrible condition, and I do think that there are some people who are more prone to it for some reason - probably genetics. However, I also think that we can set up conditions that are more likely to evoke it. It is a bit like filling a fridge with junk food, and then wondering why everyone in the house is getting fatter.
I have gone through bouts of depression. I am considered clinically depressed now and have been for a few years to the point I have medication - however, regardless of how I feel, I have the attitude that I have shit to do, so i better get on with it. It isn't pleasant.
Yeah, she is a lot like that too. I think some of it can boil down to personality. If you are a giving person you are eventually going to end up empty. Technically yes it's something you do to yourself, but saying no is so unnatural that it never occurs to you as an alternative. Despite how much it might benefit you mentally.