Although John Thornton is Buck's master, their relationship is that of equals who respect one another. When Thornton stops Hal from beating Buck, his own behavior is rather like that of a dog protecting his master (or a weaker dog). He gives a cry "that was inarticulate and more like the cry of an animal" before springing furiously at Hal and standing over Buck "struggling to control himself, too convulsed with rage to speak."
"If you strike that dog again, I’ll kill you," he at last managed to say in a choking voice.
The urbane and genial Judge Miller would certainly be disgusted by Hal's treatment of his dog but would never be viscerally moved to defend Buck with such ferocity. Perrault and François are tolerable masters by most standards, but they regard Buck chiefly as an asset and an investment. Hal, Charles, and Mercedes represent the nadir of Buck's fortunes. They are vicious and stupid, treating Buck appallingly and endangering his life along with their own.
Thornton and Buck watch together as Hal, Charles, and Mercedes fall through the ice. They look at each other, and neither thinks of saving Buck's former masters. Up to this point, Buck has been unlucky in that his masters have become steadily worse. Now, however, he has found in Thornton a friend and companion who understands him, perhaps because Thornton, in his courage and loyalty, is rather like a dog himself.
Unlike Buck's previous masters, John Thornton saves Buck from certain death, is completely devoted to Buck, and genuinely expresses his love for him on a daily basis. Although Judge Miller is a pleasant master, he never goes out of his way to cultivate a meaningful relationship with Buck. Judge Miller enjoys Buck's company but never develops a lasting bond with him. Perrault and François are Buck's next owners, who are wise and just masters. They treat Buck with respect, make sure his needs are met, and teach him how to become an efficient, obedient sled dog. Under Perrault and François's tutelage, Buck transforms into a disciplined, esteemed sled dog. However, Perrault and François are selfishly motivated to care for Buck and exploit him for his labor. Buck's worst owners are Charles, Hal, and Mercedes, who are ignorant, cruel, and hostile. Their incompetence and brutality threaten Buck's life. Fortunately, John Thornton intervenes and saves Buck's life before his three terrible masters fall through the thin ice.
John Thornton is the ideal master because he not only respects and admires Buck but selflessly cares about him and demonstrates his love and affection for Buck on a daily basis. Unlike Buck's previous owners, John Thornton is not concerned with profiting off Buck and simply has his best interests in mind. There is a genuine, inherent connection between John Thornton and Buck that both of them understand and appreciate. John Thornton is equally as loyal to Buck as Buck is to him, and both are primarily concerned with pleasing and protecting each other. John also trusts Buck and allows him to roam the wilderness, knowing for certain that he will eventually return to camp whenever he pleases. John Thornton's selfless personality, genuine love for Buck, and willingness to make sacrifices for him are what makes him the ideal master.
Thornton is different from the previous masters Buck has had in the Yukon for the obvious reason that he's kind and protective. When Buck lived in the "Southland," in the San Francisco area, he did not experience the difficult life that was in store for him after his abduction; he knew only the comfortable life of a domesticated dog in the civilized world. But Buck's new life with Thornton is better than this, and represents a kind of Eden-like existence, ended both by Thornton's death, and the inevitable transformation of Buck to life in the ancestral world of the wolf pack.
There are two factors, apart from simple kindness and decency, that set John Thornton apart from Buck's other masters, both in his previous life in San Francisco and his life in the Yukon. First, Thornton rescues him from an abusive situation, and for this, Buck gives him his loyalty. A parallel situation occurs in London's White Fang when the title character is rescued from the dog-fight arena by Weedon Scott. Second, most animal experts will assert that certain humans have an intangible quality that enables them to bond with animals in a special way. Thornton has this ability, and Buck's devotion to him is a kind of distilled form of the general trait of selfless and unconditional devotion that the dog, of all animals, is capable of granting to a person.
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.