Why does buck refuse to harnessed to the sled at first?

Buck initially refuses to be harnessed to the sled because he is not accustomed to the harsh conditions of the Yukon and the dominance of man over dogs. His instinctual pride and sense of freedom clash with the subservient role expected of him. Additionally, he is still adapting to the brutal reality of his new environment and the harsh treatment by his human captors, which fuels his resistance. Ultimately, this refusal is a reflection of his struggle to maintain his identity and autonomy in a challenging new world.