Nathan had been by Aleera's side for over a century. He could have left and started his own coven, but he lacks the talent of picking out the talented like his mistress does. Another reason he had stayed is that he had been trying to prove to Aleera that he would be worthy consort. Aleera had only fully brought him into her bed once. For the unlife of him he can't figure out what he missing in order to attract  her. "Displease my mistress and things will not go well for your sireling Dagger." Nathan had used that on purpose, reaffirming his status as being older than the snow white vampire.