cause he is a werewolf so he is hard and Bella is fragile, it also hurt her और because well he kissed when he knew she was with Edward, I would पंच him too!!
आप have to remember that Jacob was a werewolf द्वारा that time so he was extremely strong and his build was meant to withstand anything so when Bella punched Jacob there was no effect on Jacob because his बोन्स were hard to break but to Bella when she punched Jacob it was like punching a wall.