I think Jacob likes Bella so much because if Du are a boy and Du have a friend that's a girl Du will start to notice that Du have feelings for her. That's only if Du two have been Friends for a long time.
It's not really something which can be explained. People don't exactly choose to Liebe 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.
