How does Buck balance his love for Thornton with his more primitive self?In Chapter 6.
Although Buck is not yet as wild as he will become, he is already (in Chapter 6) very primitive. As the book says, the only thing that really kept him from being totally wild was his love for Thornton.
Buck shows his wild self by going around fighting with any strange dog he sees. He shows it by stealing from any other human than Thornton. And, above all, he is coming to hear the call of the wild more and more strongly in his heart.
But at the same time, he is totally devoted to Thornton. We can see this, for example, in the place where he is about to jump off the cliff because Thornton told him to and in the big thing at the end of the chapter where he pulls the heavy sled for Thornton.
So, in this chapter, Buck is becoming more and more wild and only his love for Thornton holds him in the modern world.