I think that some people have a biological disposition to be depressed, but that whether they actually develop it often has to do with how they are raised. If they have that disposition but are raised in a healthy, nurturing environment, it may never develop, and if that's not the case, it comes out. However, some cases, pregnancy, mid-life crisis, change of life issues, can sometimes throw the "biology" off enough that is appears regardless of upbringing.