The main reason that family and friends are frequent targets of aggression is that

The main reason that family and friends are frequent targets of aggression is they are very well known by these people. A lot of people will describe their feelings to anyone they are close to. The only problem with this is that when certain people get mad or upset they could turn on you when you least expect it. So, only tell the people you trust the deepest your problems and these chances of aggression will decrease

I would have to say because they are around them more often so they have more of a chance to take the heat.  hope this helps