One of the biggest reasons that Bruno does not want to leave Berlin is that it means leaving behind his three friends, Karl, Daniel, and Martin.
One of the first things we learn about these boys is that, unlike Bruno, they have a firm understanding of what their fathers do for a living. Karl’s father is a greengrocer, Daniel’s father is a teacher, and Martin’s father is a chef.
When Bruno’s mother explains to him that the family will be leaving Berlin, one of the first questions he asks relates to how he will spend time with these three friends. He struggles to process his mother’s response, which is to tell him that he will need to “say goodbye to [them] for the time being.” His mother flippantly tells him that he will make other friends. The four boys had made great plans involving “causing a lot of chaos” for the upcoming summer holidays, and Bruno argues with his mother about the move until she snaps at him and instructs him to go upstairs and help pack his things.
After adjusting to the move, Bruno realizes that the life he had lived in Berlin feels very distant, and he can “hardly even remember what Karl, Daniel or Martin looked like.” Later, he couldn’t even remember Daniel and Martin’s names. By this time, he has acknowledged that his new friend across the fence, Shmuel, means more to than his friends back in Berlin ever had.