The Call of the Wild
How we cite our quotes:
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.
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.
They stopped by a running stream to drink, and, stopping, Buck remembered John Thornton. He sat down. The wolf started on toward the place from where the call surely came, then returned to him, sniffing noses and making actions as though to encourage him. But Buck turned about and started slowly on the back track. (7.17)
Thornton is the only tie connecting Buck to the civilized world and preventing him from becoming part of the wild.