| Quote #1
"You poor devil," said John Thornton, and Buck licked his hand. (5.66)
Buck’s feelings for Thornton are immediate.
| Quote #2
Love, genuine passionate love, was his for the first time. This he had never experienced at Judge Miller's down in the sun-kissed Santa Clara Valley. With the Judge's sons, hunting and tramping, it had been a working partnership; with the Judge's grandsons, a sort of pompous guardianship; and with the Judge himself, a stately and dignified friendship. But love that was feverish and burning, that was adoration, that was madness, it had taken John Thornton to arouse. (6.3)
Buck is only able to feel the intensity of love after he has suffered hardship, not before when he resides at Judge Miller’s house.
| Quote #3
He had a way of taking Buck's head roughly between his hands, and resting his own head upon Buck's, of shaking him back and forth, the while calling him ill names that to Buck were love names. Buck knew no greater joy than that rough embrace and the sound of murmured oaths, and at each jerk back and forth it seemed that his heart would be shaken out of his body so great was its ecstasy. And when, released, he sprang to his feet, his mouth laughing, his eyes eloquent, his throat vibrant with unuttered sound, and in that fashion remained without movement, John Thornton would reverently exclaim, "God! you can all but speak!" (6.4)
London uses the word "ecstasy" both to describe Buck’s emotion at killing a rabbit and his feelings with Thornton. This suggests a connection between the extremes of love and death.