It's no accident. Your family is stuck with you, so you treat them however you please. Also, they understand you and aren't offended by your otherwise antisocial ways. Strangers, on the other hand, have to be treated with some civility and courtesy. The point of this is that familial relationships are not voluntary.
Social relationships are voluntary.
So, the family is where the psychopathology is. It's where one gets hurt before one learns to defend oneself. It stands to reason, then, that if one is going to have a dysfunctional relationship, it will conducted by familiar family rules. Strangers won't put up with that. So social rules actually are more conducive to such things as friendship and the development of intimacy.