Quote 4
For Thornton, however, his love seemed to grow and grow. He, alone among men, could put a pack upon Buck's back in the summer traveling. Nothing was too great for Buck to do, when Thornton commanded. One day (they had grub-staked themselves from the proceeds of the raft and left Dawson for the head-waters of the Tanana) the men and dogs were sitting on the crest of a cliff which fell away, straight down, to naked bed-rock three hundred feet below. John Thornton was sitting near the edge, Buck at his shoulder. A thoughtless whim seized Thornton, and he drew the attention of Hans and Pete to the experiment he had in mind. "Jump, Buck!" he commanded, sweeping his arm out and over the chasm. The next instant he was grappling with Buck on the extreme edge, while Hans and Pete were dragging them back into safety. (6.13)
Buck’s loyalty to Thornton is blind—such emotion is dangerous, and its threat to Buck’s life is counter to the survival—given laws he learned in the North.
Quote 5
Thornton knelt down by Buck's side. He took his head in his two hands and rested cheek on cheek. He did not playfully shake him, as was his wont, or murmur soft love curses; but he whispered in his ear. "As you love me, Buck. As you love me," was what he whispered. Buck whined with suppressed eagerness. (6.48)
Buck’s emotional love for Thornton overcomes physical tests.
Quote 6
The crowd was watching curiously. The affair was growing mysterious. It seemed like a conjuration. As Thornton got to his feet, Buck seized his mittened hand between his jaws, pressing in with his teeth and releasing slowly, half-reluctantly. It was the answer, in terms, not of speech, but of love. Thornton stepped well back. (6.49)
Again we see a connection between love and violence, as Buck uses a physical act to express his emotions.