When you think about it, Isaac has been doing it for a while, since the two of you started dating.

First, it’s a playful bite to your shoulder while you both were laughing about something stupid.

He doesn’t seem to notice he does it, you both could be giggling and his first reaction it to nip at you, you both could be kissing and his first reaction is to bite your neck.

Now, you are just reading while he listens to your heartbeat. He seems to want your attention and bites your jaw.