In a sense one of the main themes of the Devil Wears Prada is about belonging. I say this, because the Devil Wears Prada is about high culture fashion, which is about being part of the "in" culture. You have to wear the right thing, act the right way, and know the right people. There is an invisible code of behavior. If you do these things, you will be accepted. If you fail to do these things, then you will be an outcast. So, on the most general level, the theme of belonging is present. If you look at the work in this light, then you will be able to see the same dynamics in individual relationships.
The theme of belonging can be seen in Weisberger's work. On one level, Miranda creates a realm where there are definite practices of inclusion and exclusion, contributing to the latter's sense of wanting to belong. Miranda's presence, demeanor, style, and sense of self helps to instill a desire for belonging in Andrea. The abuses that she endures has to have some end in mind, and one can presume that wanting to belong to the world in which Andrea finds herself. Andrea cannot break free, even sacrificing personal relationships to wanting to belong to Miranda's world. When confronted with the reality that she might become the Miranda for the next generation, Andrea has realized she has reached her goal of belonging, and repudiates it, recognizing its emotionally shallow and inauthentic nature.
the theme is all about the problem of lifestyle, miranda's work as the assistant of the fashion magazine.