At the beginning of the story, Buck ran 1,000 miles. And he was tired so he decided to lay down near the river bank. Watching the running water, and listening lazily to the birds and the hum of nature. And John Thornton had to take a rest because he had frozen his feet. So John Thornton made a campfire so he can heal.
Buck slowly won back his strength. And john Thornton got back up. As well as the other dog's skeet, and nig. They were all waiting for the raff to come to carry them to Dawson.
The dogs and John Thornton was playing around and Buck realized what he was feeling. Then Buck follows a wolf, the wolf wants him to leave. But Buck doesn't leave because he loves John Thornton.
John Thornton bets that Buck can carry 1,000 pounds on a sled. And Buck does it because he loves John Thornton.
Buck earned John Thornton money from pulling the sled with 1,000 pounds. When most dogs couldn't do it.
Buck was feeling love; genuine & passionate love. He was being loved and he felt safe with John Thornton. He was feeling like he was getting treated fair. And the other dogs weren't jealous of Buck.