Shape-shifters can change at will, and werewolves are just slightly different, having to change at the full moon. When Bella finds out that Jacob loves her, she has conflicting feelings. She is still madly in love with Edward, but it looks like Edward has left and will not return. Throughout the whole book of "Eclipse" Bella is trying to decide if she should give in to Jacob and pursue a relationship that would be good, but not better than her one with Edward. Something inside her tells her that Edward is the only one for her, but she sees the devotion in Jacob and has a flash of what their life could be like together. If she decided to be with Jacob, she could live a more human life, have a family, and she knows that Jacob would love her. However she also knows that she'll always love Edward, and that would be unfair to Jacob. Obviously, Bella does not end up with Jacob, but these are the conflicting emotions that run through her mind when Jacob falls in love with her.