So me and my girlfriend were making out and we started biting eachoters lips, why does that feel so gooood?

Well it was for like an hour that we made out so we kept getting bored and thinking of new things to do and idk how it happened but I accidently bit her lip and she moaned lol. So I kept doing it and she did it to mine. Why is this so stimulating lol? And any other ideas to please her while making out?