The two men find it hard to harness Buck to the sled at this point because they are trying to put him in his usual place, which Buck is no longer content with. He has just triumphed over Spitz and now covets Spitz's position as leader of the sled team.
The men don't understand Buck's wishes and at first choose the older, more experienced dog Sol-leks as the leader. Buck will not stand for this and physically pushes Sol-leks aside.
Buck was in open revolt. He wanted ... to have the leadership. It was his by right. He had earned it, and he would not be content with less. (chapter 4)
Buck, then, defies the men, staking his claim for top spot in the team, which he has 'earned' by disposing of Spitz.
Finally, the men have to give in to Buck's wishes. He takes his place triumphantly at the head of the team and commands so well that the men are in awe of him. Buck is the true hero of the story and no other dog can match his qualities.