How is John Thornton different from Buck's previous masters? Why is he an ideal master?
Judge Miller, Buck's original owner in California, was a responsible and caring individual with whom Buck had a very good life. When Buck was stolen from the judge, his life changed radically.
Francois and Perrault depend on their sled dogs, including Buck, for their living. They are not unkind to the dogs but demand a great deal from them, driving them long and hard to get the mail delivered, and do not always provide the food and rest needed for full recovery from the mail runs.
Charles and Hal do not understand how to survive in the Far North and act out of ignorance. They beat and starve the dogs and drive them far beyond their capacity because they don't understand the needs of the animals and are driven only by their human greed for gold.
Judge Thornton treats Buck with caring and respect, allowing Buck to develop a relationship of trust and loyalty based upon recognition of Buck's needs and meeting them - the basis for any strong relationship. Buck responds with devotion and all the protective instinct he possesses.