There is some MI's that are passed on like bipolar or schizophrenia & some types of depression.

HOWEVER, these same MI's can be induced by illegal drugs, or situational experiences. My anxiety was TOTALLY SITUATIONAL. When I got out of the situation, that depression, anxiety & suicide attempts ended completely & so did the anorexia I was struggling with.

Pachy is right....there is also a bio psychosocial model that explains a lot of what can be causes also.

The reasoning is not simple or straight forward & can't be given a simple explanation as to where MI comes from in a person