I think Jacob likes Bella so much because if Ты are a boy and Ты have a friend that's a girl Ты will start to notice that Ты have feelings for her. That's only if Ты two have been Друзья for a long time.
It's not really something which can be explained. People don't exactly choose to Любовь someone else. Some Фаны theorise that he was drawn to her because she was related to his future imprint though, similarly to how Sam dated Leah before imprinting on her cousin.