Buck is accepting of John Thornton because John treats Buck with love, honor and respect. Buck had been taken from his home, abused, beaten, passed from one abusive master to another and had lost all faith in humans. When John comes along and buys Buck from yet another abusive human he is at first very leary of John, but Buck finally learns to trust his new master and they develop a mutual respect for each other, if not complete dedication. John does not try to keep Buck from wandering, he does not hit him, he does not starve him and he lets Buck be as free as Buck needs to be.
"Thornton "was the ideal master. Other men saw to the welfare of their dogs from a sense of duty and business expediency; he saw to the welfare of his as if they were his own children." Buck becomes his loyal friend and loves him more than any human or beast he has ever known."