The relationship between Buck and John Thornton symbolizes the bond between animals and humans. John is the pull that keeps Buck from going wild. When Buck lives with the judge, he lives like a king. Everything is his domain, but he is not especially close to anyone. All of the humans from there on are exploitative, ignorant, or abusive. Then Buck meets John Thornton, and it is a relationship based on love and trust.
When John Thornton first comes across Buck, he sees him with a group of people who do not know what they are doing. They are clearly going to kill themselves and the dogs. Wanting to follow the code of the North, Thornton doesn’t intervene at first. He just looks on in anger. Then he finally can’t take it anymore.
John Thornton stood 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. (Ch. 5)
John Thornton does nothing to stop the people from meeting their fates. He cares more about the animals than the humans. More importantly, he has seen that these people will succumb to their stupidity sooner or later. He hates what he has seen them do.
The relationship John Thornton has with Buck is pure heaven after what Buck has been through. It would be so easy at this point for him to just run off, but Thornton is the polar opposite of the type of people that Hal, Mercedes, and Charles represent. They are abusive and ignorant. John Thornton loves and appreciates Buck. Buck loves him back.
The relationship that Buck and John Thornton exemplify is different than the one described in the beginning, where Buck helps the judge’s sons hunt and has an appreciation for the judge and what the judge provides. It is Buck in his place, ruling his kingdom and household. With John Thornton, Thornton and Buck are one another’s everything.
Love, genuine passionate love, was his for the first time. This he had never experienced at Judge Miller's down in the sun-kissed Santa Clara Valley. With … the Judge [he had] a stately and dignified friendship. But love that was feverish and burning, that was adoration, that was madness, it had taken John Thornton to arouse. (Ch. 6)
It is important to note these different roles dogs play in people’s lives. For some people, a dog is a pet. For some, he is a business partner. For others, he is a family member. The relationship that John Thornton and Buck have exemplifies the term “man’s best friend.” It is a real bond.