The correct answer is Buck’s love for Thornton. There is bet placed about Buck’s strength, stating that he will not be able to pull a thousand pound sled. The amount is huge: 1600 dollars. Buck is able to do it and win the large amount for John Thornton. Although as a dog he really does not even know what is going on, his instinct tells him that accomplishing this task will please Thornton and because he loves him that makes him happy. This increases their bonding and loyalty to each other.