Despite their being from different countries and having entirely different cultures and religions, Bruno and Shmuel, who do share a birthday, are both young boys who miss their first homes, and, as friends, they wish they could play and be happy again because they are imprisoned, either in reality or figuratively.
When Bruno first meets Shmuel, he does not realize that the frail, quiet boy is imprisoned in the concentration camp of Auschwitz ("Out-With" as Bruno calls it) and longs to be home in Poland. He, too, is discouraged and writes to his grandmother about how unhappy he is there in "Out With," and not in Berlin. Nor does Bruno realize that Shmuel and the others there are not clothed in some type of pajama, but are, instead, wearing a ridiculous prison uniform. Ironically, Bruno believes that Shmuel has an advantage over him because he has many boys with whom he can play, while Bruno does not.
"It's so unfair. I don't see why I have to be stuck over here on this side of the fence where there's no one to talk to and no one to play with and you get to have dozens of friends and are probably playing for hours everyday. I'll have to speak to Father about it."
But, Shmuel, who understands the realities of where they are, cannot reveal the truth to Bruno, the son of the Commandant. Instead, because he is Bruno's friend, he pretends that they are not so different so he can have a friend to talk with and see. Regrettably, Bruno lacks the character of Shmuel because when Schmuel is caught chewing by Lieutenant Kotler, he denies having given the boy something to eat, and Shmuel is beaten later on. Nevertheless, Schmuel accepts Bruno's later apology. And, when Schmuel is worried about his father, whom he has not seen return from a "march," Bruno dons a pair of the striped pajamas and walks with Schmuel, hoping to find his father only to die with Schmuel and sacrifice his life for his friendship.