Buck has developed into a primitive beast with strong instincts during his time in Alaska. Under the conditions of trail life and living as a sled dog, this instinct grew stronger. Spitz knew that Buck was his adversary and never missed the chance to show his teeth to Buck and bully him.
"He (Spitz) even went out of his way to bully Buck, striving constantly to start the fight which could end only in the death of one or the other." (pg 33- chapter 3)
The two of them went out of their way to provoke each other. Spitz stole Buck's sleeping nest, and he attacked him when they were trying to run off the wolves. But Buck was intelligent and wouldn't be provoked into a fight. He....
"...could bide his time with a patience that was nothing less than primitive." (pg 42 - chapter 3)
It was inevitable that the two dogs should fight for leadership of the sled-dogs. It was part of their nature - part of the pride of trail and trace. This pride made dogs perform in dog sleds until their last breath. The worst thing you could do to a sled dog was take him out of the harness because he was not pulling his weight.
Buck came between Spitz and his team. If Spitz went to punish one of the dogs, Buck would get between them. There was general insubordination among the dogs incited by Buck. Spitz was losing control, and he knew it.
Francois, who is the dog-driver, was aware of the tension between the two dogs, and in the land of survival he knew that a life-and-death struggle had to take place between the two of them. It was part of their survival instinct.
When they were camped one night, a snow rabbit ran across their paths. The dogs all gave chase. Buck was stirred with
"....the blood lust, the joy to kill - all this was Buck's, only it was infinitely more intimate. He was ranging at the head of the pack, running the wild thing down, the living meat, to kill with his own teeth and wash his muzzle to the eyes in warm blood." (pg 48 - chapter 3)
Spitz beat him to the rabbit and killed it. Buck did not slow down. He hit Spitz so hard that he missed his throat. The time had come for the life-and-death fight. It was the instinct of the primitive beast that had gotten aroused in Buck during the rabbit chase that carried Buck to the final stages of his fight with Spitz.
"Buck stood and looked on, the successful champion, the dominant primordial beast who had made his kill and found it good." (pg 53- chapter 3)