just because he loves bella doesnt mean he'd imprint on her...just like sam and leah, they were in Liebe until he found emily. The wolfs imprint on someone who is most likely going to carry on the wolf gene and be a strong genetic match. Renesmee is half human half vampire, which means if her and jacob had Babys they would be 1/3 human, 1/3 wolf and 1/3 vampire. They would be shapeshifter like their dad but they would have vampire qualities- such as inpentrable skin, speed and extra strength- making them pretty much invincable i.e renesmee would produce the best offspring with the best genes
He doesnt have control who he imprints on...just like Sam and Leah..Sam thought he was in Liebe with Leah until he saw Emily...and *boom* the imprinting happened. It has something to do with 'soulmates' and making the bloodline stronger for the future wolves.