Why is what Buck does unusual at the end of Chapter 5 in "Call of the Wild"?
At the end of Chapter 5, Buck licks the hand of John Thornton. His unprecedented action signifies the beginning of "a love that was feverish and burning, that was adoration, that was madness" for a man that the dog had never experienced. He had been fond of his first master, Judge Miller, but John Thornton has aroused in him a devotion and loyalty that transcended life and death.
Buck had refused to go on with Hal, Charles, and Mercedes partly out of exhaustion but partly too because he had sensed "impending doom...of the thin and rotten ice he had felt under his feet all day, it seemed that he sensed disaster close at hand, out there ahead on the ice where his master was trying to drive him...(so) he refused to stir". When Thornton delivered Buck from Hal's relentless blows and the two watched as the team and crew went on to their doom upon the cracked ice, Buck understood in an uncanny way that Thornton had saved his life. This, coupled with the fact that Thornton, with his gentle ways was "the ideal master", enabled Buck to embark upon new territory - the experience of living for the love of a man (Chapter 5-6).