Second, Buck can learn to fight with other dogs. Because Buck saw Curly’ s death, so he thinks he must be strong to protect himself and can't show weakness, because it will make him to be bullied by the other dogs.
He fights back and never give up, he does everything to survive. He thinks that if he is strong and powerful, other dogs would not dare do anything to him. After that, Buck knows how to fight and he wants to be a leadership. So he must get rid of Spitz.