I think Jacob likes Bella so much because if you are a boy and you have a friend that's a girl you will start to notice that you have feelings for her. That's only if you two have been mga kaibigan for a long time.
It's not really something which can be explained. People don't exactly choose to pag-ibig someone else. Some fans 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.
