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Add YoursBuck is full of passionate love for John Thornton, who not only saved him, but is also the ideal master. His dogs are his children. John talks to them for long periods of time. He grasps Buck's head and rests his own upon it. It is almost as if he understands what Buck says to him. But Buck does not seek these tokens of affection. He is content to adore from a distance. At times he does not want to let John out of his sight, fearing that he will lose him as he has lost other masters.
Despite this great love, however, the "strain of the primitive" remains alive and active in Buck. He is a wild dog who sits by John Thornton's fire. He has no qualms about stealing from other camps, even though he leaves John's supplies alone. Buck gets into fights, and he fights shrewdly and with cunning. His lessons from Spitz are not forgotten. He knows it is still "kill or be killed." Inside Buck lives all the dogs that came before him. Each day mankind slips further away; Buck roams the forest, only love for John Thornton bringing him back.
Source(s)
http://www.gradesaver.com/call-of-the-wild/study-guide/summary-chapter-6-for-the-love-of-a-man