Well, I strongly believe that environmental factors are only one of many ways a person can develop mental illness. I believe some of it may be genetic predisposition as well. My mother and her sisters and their mother all have suffered varying degrees of it, one way or another. Whether they'd ever admit to it or not, in the case of my grandmother, who was too proud to admit she had anything wrong with her mental illness wise. I think it was more so the stigma from her time. A different era I suppose.

So yeah, perhaps its any given number of factors that contribute to MI.