Actually, this behavior is quite normal. Cats are all different. Their personality can be mean, loving, or a combination of both. Many will nip at their people, I tend to call them love nips, but each has their own reasoning. My cat will sit on the back of my chair and chew on my hair and scalp. No reason, he just likes to nip. I also have one that nips at me when I am petting him as if he is trying to tell me he likes it. As long as he doesn't hurt you, it really shouldn't be a concern. However, if the bites become painful or starts to draw blood, you might want to see if there is a pattern--like the presence of another cat, an injury where you are petting him, you have played with him using your hands and not toys, the list can go on and on. Just remember, your cat will not change from nice to not nice without a reason.