Here are some of the reasons why the ending of The Call of the Wild is bittersweet:
* Buck finds happiness and freedom in the wild. After spending much of the novel being mistreated by humans, Buck finally finds a place where he can be himself. He is free to roam the wilderness, hunt for food, and live according to his own instincts.
* Buck loses his human companion, John Thornton. Thornton is a kind and compassionate man who treats Buck with respect and kindness. He is the first human that Buck has ever truly loved, and his death is a devastating blow.
* Thornton's death is necessary for Buck's personal growth. Thornton's death forces Buck to face his own mortality and to come to terms with the fact that life is full of both joy and sorrow. He emerges from this experience as a stronger and more independent creature.
Overall, the ending of The Call of the Wild is a complex and nuanced one. It is a story of both loss and redemption, and it ultimately leaves the reader with a sense of hope.