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

Family and friends frequently become targets of aggression is that "they're there", that is, they are present when you come home from work, or when something happens and might just accidentally encounter us in a bad mood. 

It would probably be because they are always there in good times or in bad times. Say you had a bad day at work and you just want to be alone in your room listening to music, it is natural that they want to ask why. You want to be alone and aggression happens where you yell at them to leave you alone and you get in a bad mood.

3. i would say one, but one is not prime nor composite.