Habit, unlike the regular people done on earth, he was anything but that, he was an animal in a human constum that he wore to hurt, and kill the earth's inhabitants. But since he never advertised that he was an animal, he wasn't expecting for you to learn the weaknesses he had for being a wolf. So when you started petting his head, bot only did it feel good, but god he hated that you had that information, that knowledge over him. Made him feel weak and vulnerable to such a pathetic mortal