I don't know if you've actually read any of the credible results from the google query you link to, but there being environmental factors (including triggers and contributing factors) to depression does not mean that depression is a response to external events in the way that "normal"/healthy emotions are - which is what I'm getting at in the previous comment.
You might want to take a look at what the American Psychological Association and the American National Institute of Mental Health have to say about the case [0, 1], the University of Maryland's patient education article [2], Saveanu and Nemeroff's paper "Etiology of depression: genetic and environmental factors" [3], and clinical criteria (like DSM and ICD).
