A big part is genetics. If a rooster is agressive, odds are his sons and grandfather's are as well. Of course sometimes a bad one pops up from good males and a good one pops up from bad males (we have had both personally)Does this behavior come from not being socialized? I ask because, I had a bad rooster who used to attack my daughter. We didn't spend a lot of time with that flock and he was very dedicated to protecting his girls. The young boys we have now may just be too young to start this behavior, but they seem better, we've been very 'close' to them since they hatched. I did see slight 'aggression' in one (of the two) when the girls first started laying a couple of weeks ago, but not since. They are very used to us.