Was having a discussion with my husband about this. Genetics matters - some breeds seem to have more 'male' dominance, mating and aggression. But I've had two related roosters of the same breed be night and day- one aggressive and one sweet. Hmm. The number of hens he is protecting matters -don't give him too many- 6 or so seems ideal. Personally I think that a rooster learns manners from the older roosters around, and if there isn't a good roo role model for him to follow around at a young age, he is more likely to be aggressive being the only protector around. If his women are able to be taken from him by other roosters around that he doesn't trust, it stresses him out daily trying to keep them as his. Also, my roosters only ever fight (and not too bad) when they are locked up for a couple days, otherwise one will normally run away and chill alone somewhere when provoked, and sneak his way back into the flock later, without the fight having to happen.