Why should readers sympathize with Buck throughout The Call of the Wild? Why should they not?
The reader can certainly sympathize with Buck when he goes from his privileged and easy life with the Judge to the rough and tumble life of the trail. Buck developed the ego to go with his unofficial position as the king of the estate, but when he is stolen and taken away by the gardener, it is easy to feel sorry for him as he learns to deal with physical abuse and the privations of the trip north.
It is also easy to sympathize with Buck because he lives with a sense of justice and fairness that is appealing to the reader. He makes mistakes but learns quickly and there is a sense that the brutality and violence of the dogs in the Yukon is a reflection of the harsh reality of life there for humans as well.
Because Buck becomes a ruthless killer, it can also be necessary not to sympathize with him, even if the conditions seem to justify his ruthless and brutal actions. Buck also begins to separate himself from the world of men and the loyalty to instinct overcomes his loyalty to mankind, something that can also lessen a reader's sympathy for him.