Buck shows his love for John Thornton by twice saving his life and by obeying him when he tells him to jump over a cliff. Thornton, in turn, loves Buck and does not abuse him like the other men had. There is a genuine respect and love between them, and Buck senses this. Unfortunately, Thornton is killed, but Buck continues to show his love for him by visiting his grave each year.
Buck showed his love for John Thornton because he stood up for John in the saloon when he was in the fight with the man. He saves Thornton's life twice. Also Buck only returned to the camp because he loved John Thornton, and there was a friendship still to be had. After Thornton dies he tries to fight off the Indians because they had just killed his best friend, and he wanted revenge.