What was Bruno's reaction to the people in the striped pajamas?

Bruno's reaction was curiosity. Who were these people? Why were they on the other side of the fence? When Bruno sees Shmuel he notices both the pajamas and the symbol.... the Star of David, though he doesn't know what it is. Bruno's reaction is to think that he is "sure that he had never seen a skinnier or sadder boy in his life" (107).