This is fairly interesting to assess. I am not sure if there is a singular answer to it. On one level, there is an intense desire to belong on the part of the Nazis. As the rhetoric and propaganda increase, the Nazis are shown to be individuals who crave belonging to and with one another. Gretel's budding sexuality is merged towards the ideal Nazi soldier, the personification of the ultimate human being in accordance to Nazi ideology. At the same time, Bruno's father is driven to "belong" to the group of individuals that have power and to be part of that inner circle of Nazis who are to represent "the best" in everything. While this belonging is of the temporal, there is another sense of belonging in the friendship of Bruno and Shmuel. They share a belonging to one another which lasts through and defeats contingency and that of the temporal time period. We see this when Bruno betrays Shmuel. The shame he feels and acknowledgement of their friendship afterwards is a moment of belonging where the permanent has transcended that of the temporary. This becomes heightened when Bruno helps Shmuel find his father. It is here where the theme of belonging to one another is something that causes action to be taken in the name of solidarity with consequences that are both horrific, but also call upon the better angels of human nature.