This answer is subjective and left up to individual readers. The book is a young adult novel, so the author focused on finding ways to make his main characters sympathetic to a particular reading audience. In this case, that audience is young teenagers.
One immediate way to make characters sympathetic to young readers is to have young protagonists. That is the case in this book. Bruno is nine years old, and that happens to coincide with Shmuel's exact age as well. Even adult readers are likely to sympathize with those two characters because adults are likely to remember what it was like being that age.
The story's setting isn't likely to have been experienced by any current readers, so the author puts his characters in situations that resonate with modern-day audiences. For example, Bruno being angry at the move and being basically friendless is likely to elicit sympathy from readers. Readers are also likely to sympathize with Shmuel's loss of family members.
A result of the author...
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