Buck followed John Thornton everywhere. "From the moment he left the tent Buck would follow at his heels." Buck felt a warm love for John Thornton, and he feels that this is one of the first humans he can completely trust and depend on.
Buck is having an inner conflict with himself on whether he stays with John Thornton and returns to civilization or if he wants to continue on with his new wild life. But, in the end he realizes that he feels a special bond with John, and he forgets about the call of the wild.
An evil man by the man of "Black" Burton, was picking a fight at the bar, and John stepped into the fight to stop them, while Buck was watching this. When John was knocked back, Buck roared and threw himself on Burton, and ripped out his throat, but then he was taken away from the man. This gave Buck a reputation that was known about across all of Alaska.
Buck saves John Thornton from the worst part of the rapids, and this secures John Thornton's trust of Buck and shows how much love and care Buck has for John. Buck has now saved John from death for the first time.
John Thornton bets that Buck can pull a 1,000-pound load, and he bets 1,000 dollars on it. This makes him very nervous and he depends on Buck to save him out of this sticky situation.
Buck pulls the 1,000-pound load to everybody's surprise, and John Thornton cries with happiness, and shakes him back and forth, while the onlookers create a respectful distance between the man and his best friend.