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.

Buck has gone through a horrible series of events by the time he ends up with Thornton.  He has had masters that he could trust, and he has had ones that were horrible beyond belief.  When Thornton arrived on the scene and saved Buck from being beaten to death by the woman, her husband, and her brother, Buck realizes that he has "landed on his feet," so to speak.  When Buck licks Thornton's hand, he is letting him know (and us, the readers) that he trusts him and believes in him, and even that he will lay his life down to defend him.

Check the links below for more information on these characters!  Good luck!

Buck has been kidnapped from a loving home and treated abominably by humans. In his short life, he's already seen both the good side of man—when he was playing happily on the ranch—and the bad side—when he was abducted and shoved aboard a train heading for the frozen north. Given what's happened to him, it would be no surprise if Buck never trusted another human being as long as he lived. That he does learn to trust humans again is due in no small part to Thornton. It's Thornton who saves Buck's life, and whose kindness and consideration help to restore Buck's damaged faith in humanity. Although Buck, through his various adventures, becomes more and more in touch with his wolf ancestry, he'll always retain a spark of the domestic animal that once playfully gamboled around Judge Miller's ranch without a care in the world. And the kindly Thornton is mainly responsible for this.

In the novel The Call of the Wild, Buck has gone from being a beloved pet, to a hardened dog capable of withstanding almost anything. When Thornton rescues Buck, the dog comes full circle to truly love a human being again.  Buck loves Thornton, which shows that he is still capable of being able to love after all of the abuse, that he is willing to give up everything for the love of this man and stay with humans despite listening to the wolf call.  When Buck is out of the camp and comes back to find Thornton and all of the others dead, he gives up his desire to stay with humans, meaning Thornton, and answers the "call of the wild".  He has now truly become a creature of the wild just like the wolf and returns to his ancestral state of a wild, undomesticated creature.