I'm not sure, but I believe some are born with a disposition toward developing depression.
In my case MI is on my mom's side. So, there's likely a genetic component also. If you add environmental factors to the equation it's a recipe for acquiring depression IMHO.

I look at old pictures and interestingly from 0-5 I look happy and am smiling a lot. Not sure, but I strongly suspect that when my little baby bro came along my probs with depression began. My mom has shared with me in confidence how she was not expecting to get pregnant again after having 3 kids. But, much to her surprise she became pregnant during her cycle. Number 4 came along and it became emotionally & physically overwhelming for her to care all of us. I went from 'the baby' to a middle child with an emotionally unstable mom. It makes sense to me and pictures from my childhood support my theory.