Thornton was attacked by an evil-tempered man named Burton, and which Buck came back to attack him to defend his master.
Another time was when Thornton slipped off the raft and into some rapids, and Buck jumped in to save him. When he came back out he had cuts and bruises with three broken ribs.
The last time he saved him was when Thornton was bragging about Buck and said that his dog alone could pull 1000 lbs. So he got himself into a rushed bet and didn't know if buck could pull that weight. But since Buck had a strong love for him, Buck successfully started to pull the sled out of the ice and onward 100 meters.
Buck in chapter 6 has had many memories of his past.
First Buck thinks about his last masters, and how they have all passed out of his life at some point. Now Buck is afraid to lose Thornton.
Buck also remembers the valuable lessons he was taught in his past: First was clearly the Law of Club and Law of Fang. Secondly, he learned how life works for him such as master or be mastered; and to show no mercy which hadn't existed in his primordial life, kill or be killed; eat or be eaten, for this was the law.