I think that Buck changes because he learns to fully grasp the nature of human cruelty. Buck undergoes changes from a domesticated dog to a wild one as a result of all that is done to him. His abduction from Judge Miller was the result of human deceit and through this, Buck learns how to adapt in the wild. Buck's loyalty to Thornton is once again severed by human cruelty. This causes him to change into a being of the wild to forgeo the world of humans, fraught with disloyalty and dishonor. The notion of civilization being more uncivilized than any other domain helps to bring out the changes in Buck. He understands the rules of the wild as the dreive to survive is the only adversary. These rules are clear, while the rules that govern the world of humanity are far from clear and obscure, at best. The refined coat he has at the start of the narrative is replaced by one that is weathered more by survival and the wild. This helps to reinforce the emotional change that Buck undergoes, from one who enjoys what is deemed as luxury to a survival based existence where trial and challenge exist at every turn. The lure of the wild in terms of the hunt and the nature of "kill or be killed" is something of which Buck becomes a part, fully evolving into the "Ghost Dog," a being of the wild.
Buck changes because in chapter 1 he was treated well, bu in chapter 2 in his encounter with the man in the red sweater he had learned that there are such things as cruelty leading to him transforming from a peaceful dog to a raging animal.
Hope that Helps!