Why Does the book of the call of the wild have a sad ending?

The ending of "The Call of the Wild" is sad because it highlights Buck's transformation from a domesticated pet to a wild creature, ultimately embracing his primal instincts. As he answers the call of the wild, he leaves behind the human companionship he once cherished, including John Thornton, who represented love and safety. This poignant departure underscores the themes of survival and the harsh realities of nature, emphasizing that true fulfillment often comes at the cost of personal loss. Buck's journey reflects the bittersweet nature of existence, where freedom and instinct sometimes require the sacrifice of cherished bonds.