How were Buck's feelings for Thornton different from his feelings for his previous masters?

Buck feels a strong connection with Thornton, his final master, and is deeply devoted to him. This is new for Buck: toward his previous masters, Buck had felt a kind of angry revulsion. If Buck obeyed his previous masters, it was because he had to. But Buck obeys Thornton out of a genuine wish to please him and protect him.

As Buck's first real masters after he was kidnapped from the Judge's house, Perrault and Francois are reasonable with Buck. Still, they're the reason for Buck's harsh new lifestyle, and even if he holds a thin respect for these men, Buck certainly doesn't love or admire them.

Compared to Perrault and Francois, Hal and Charles are terrible masters: cruel and greedy, with unrealistic expectations of how far the dogs should run and how little they should eat. Toward these inexperienced and vile men, Buck feels only irritation and outrage. His entire experience with them leaves Buck more jaded and exhausted than ever:

Late next morning Buck led the long team up the street. There was nothing lively about it, no snap or go in him and his fellows. They were starting dead weary. Four times he had covered the distance between Salt Water and Dawson, and the knowledge that, jaded and tired, he was facing the same trail once more, made him bitter. His heart was not in the work, nor was the heart of any dog. The Outsiders were timid and frightened, the Insiders without confidence in their masters.

You can tell that things will be different with John Thornton when he steps in rescue Buck from the beatings that the cruel Hal and Charles are inflicting:

The last sensations of pain left him. He no longer felt anything, though very faintly he could hear the impact of the club upon his body. But it was no longer his body, it seemed so far away. And then, suddenly, without warning, uttering a cry that was inarticulate and more like the cry of an animal, John Thornton sprang upon the man who wielded the club.

Buck even licks John's hand on that day, a rare sign of affection. It doesn't take long for the bond between the dog and this man to grow into genuine love:

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.