You do not really tell us where in the book you are talking about, so I'm not sure that I can give you a precise answer.
However, over the whole book, Buck starts to feel more and more like a wild dog. This is really the main story of this book -- it is about how Buck goes from being a civilized pet to being a wild animal.
When Buck first gets to the wild, he has to learn how to be vicious and wily. He learns about how to act in a fight and how to survive in the cold and how to steal food. As he grows stronger and more competent, he starts to hear the "call of the wild" and he becomes more and more like a wild animal.
Finally, at the end of the book, he abandons humans altogether and goes to live with the wolves.
Buck is a well bread husky. In his first household he is 'the king', the other dogs respect him and he was used to this lifestlye. When he is first shipped to the place where he is sold he is put into a group where he isn't top dog anymore. The book's main storyline is how Buck becomes a wild dog, but also his rise to the top of the chain is a main theme in the book. As Buck becomes stronger and stronger he gets the urge to be in the wild more and more and soon becomes one of the wolf pack. Towards the end of the book its just him and his master out in the wild, and when he runs off with the other dog/wolf we get a sense he is complete and is fulfilling his destiny. The whole way through the book he is faced with challenges he will have to face on a daily bases in the wild so in a way the whole book represents his journey to becoming a wolf, and almost evolving backwards to his first primeval state.