I agree with silkiechicken that it is largely the roo to begin with; however, establishing yourself as top roo makes a big difference as well. If the roo thinks he is the boss of you, he is likely to be more aggressive towards you. Gritsar has a great member page on rooster reform. I would recommend reading it (if you see her post, click on the member page link under her avatar). While you don't need to reform any roos, it gives great insight into how a rooster thinks.