Quote 7
But Thornton fell on his knees beside Buck. Head was against head, and he was shaking him back and forth. Those who hurried up heard him cursing Buck, and he cursed him long and fervently, and softly and lovingly. (6.59)
While Buck bites Thornton to show his love, Thornton curses him. There is a painful aspect to their love.
Quote 8
John Thornton was eating dinner when Buck dashed into camp and sprang upon him in a frenzy of affection, overturning him, scrambling upon him, licking his face, biting his hand--"playing the general tom-fool," as John Thornton characterized it, the while he shook Buck back and forth and cursed him lovingly. (7.18)
Although Buck’s loyalty to Thornton may waver for moments in the wild, he never loses his devotion.
Quote 9
"Never was there such a dog," said John Thornton one day, as the partners watched Buck marching out of camp. (7.23)
Thornton recognizes Buck’s true value.