In The Call of the Wild, the bond between Buck and Thornton is best explained by observing that

the bond is Buck's last and strongest tie to civilization.

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http://www.sparknotes.com/lit/call/canalysis.html

In order to explain the bond between Buck and Thornton in The Call of the Wild, we need to observe their interactions and examine the events that unfold throughout the story.

To start, let's look at Buck's journey in the novel. Buck is a domesticated dog who is thrust into the harsh and unforgiving world of the Alaskan wilderness. As he adapts to his new environment, he undergoes a transformation from a spoiled pet to a powerful and independent sled dog.

During Buck's adventures, he encounters a variety of humans, some of whom mistreat him and view him merely as a means to an end. However, when he meets John Thornton, everything changes. Thornton is different from the other humans Buck has encountered. He treats Buck with kindness, respect, and understanding, recognizing the dog's true nature and potential.

Thornton provides Buck with love, care, and companionship. He understands Buck's needs, both physically and emotionally. Thornton also saves Buck's life multiple times and nurtures his loyalty and devotion. This experience greatly impacts Buck, as he forms a deep bond with Thornton, which becomes the central connection in the story.

The bond between Buck and Thornton is best explained by the mutual trust, respect, and genuine affection they develop for each other. Thornton represents the embodiment of a true companion, one who values Buck not just for his strength and skills, but also for his inherent spirit and loyalty.

In observing their relationship, it becomes evident that this bond is characterized by Buck's unwavering loyalty and devotion to Thornton. Buck is willing to do anything to protect and please his beloved master, showcasing the strength of their connection.

Therefore, the bond between Buck and Thornton in The Call of the Wild is best explained by observing their interactions, the care and kindness Thornton provides, as well as the loyalty and devotion Buck demonstrates throughout the story.