In Buck’s eyes, which of John Thornton’s qualities make him the ideal master?

From the text:

Love, genuine passionate love, was his for the first time.

Other men saw to the welfare of their dogs from a sense of duty and business expediency; he saw to the welfare of his as if they were his own children, because he could not help it. And he saw further. He never forgot a kindly greeting or a cheering word, and to sit down for a long talk with them (“gas” he called it) was as much his delight as theirs. 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!”