John Thornton was camping alone because while navigating the North the previous December, he had frozen his feet. His companions, realizing that he was unable to go on with them, "had made him comfortable and left him to get well, going on themselves up the river to get out a raft of saw-logs for Dawson". By the time Buck came into his life, Thornton was much improved, limping only slightly, and with further time for recuperation in the spring, he and Buck gradually regained their strength and were ready to travel again when the partners returned.
John Thornton was the ideal master because he of his kind and generous nature. While "other men saw to the welfare of their dogs from a sense of duty and business expediency, he saw to the welfare of his as if they were his own children, because he could not help it". Thornton sincerely cared about his dogs, and spending time with them "was as much his delight as theirs". He took interest in his dogs in a way that transcended just what they could do for him; he got to know them as individuals and treated them with dignity and respect (Chapter 6).
Thornton was left behind by his friends because his feet were frozen and he was unable to travel. They "made him comfortable and left him to get well, going on themselves up the river to get out a raft of sawlogs for Dawson." It's here by the river that Thornton and Buck both convalesce. It seems to me that their joint healing is an example of Thornton's fitness to be Buck's master. London writes that "other men saw to the welfare of their dogs from a sense of duty and business expediency; he [Thornton] saw to the welfare of his as if they were his own children, because he could not help it." The idea that Buck somehow is Thornton's child is not really an expression of anthropomorphism. Buck is a "creature of the wild" and responds the call of nature. Thornton's love, the secret to why he is such a great master, is based on the idea that he loves Buck as a dog. Their bond is so powerful because each understands the other for what they are, man and dog. It is a bond of real love. Buck "knew no greater joy" than his master's "rough embrace" -- Buck's joy in his master's companionship was so great, Thornton remarks to himself, “God! you can all but speak!”