I suspect that depression is a combination of several factors one of which is biological predisposition. One's circumstances and experiences determine whether depression is triggered or not.
For myself, I know I have a predisposition but since my two former biggest stressors (poverty and my mother) have been removed, my struggle with depression has been greatly reduced. Lack of stressors alone would not be enough, but I feel spiritually connected to God and emotionally connected to my husband and children and that fills up my purpose void pretty well. I still get some SAD going on, but nothing like I used to get.
*Warning: anecdotal crap*