I believe its a genetics thing combined with environment. My dad was what they used to call " manic depressive". He has substance abuse issues. My parents' marriage was full of fights and cheating and I have ptsd from it. The lack of consistency and chaos kept me from feeling safe. So I have a predisposition to mental illness and a bad environment and I have BPII among other things. I do not believe having the predisposition alone would have made me mentally ill and I do not believe environment alone caused it.