For me, Genetics, than nature and a specific life event - job loss, caused my depression.
Once the job loss happened, what followed was social isolation and loneliness, what followed that was severe Depression. That was in 2009. I've been suffering ever since,
despite using 2 SSRI's and Abilify, and Omega-3 Fish Oils, and Vitamin-D. The Depression doesn't seem to want to lift despite what I do.