Different social classes.
From the beginning of this book, there is a clear distinction between social classes that accompanies the characters throughout the book. The higher social classes, such as the count, the rich trader and the rich wine merchant behave differently from the lower social classes, such as Boule de Suif. During the journey, they take items that will make their trip comfortable and plan to eat in the restaurants, while Boule de Suif takes with her only food for her sustenance. 0
The rich people refuse to be equal to lower social classes and that is why they don't talk to Boule de Suif; they think a prostitute shouldn't sit among them. They see her and people like her with contempt, as dirty creatures who aren't worth much and have no real purpose in life. Even when she saves them by sacrificing herself, they don't appreciate her help and see her with disgust.
Manipulation
Another noticeable theme in the book is the theme of manipulation, which is closely related to self-interest. When Boule de Suif and her companions are held captive by the Prussians, all her companions try to manipulate Boule de Suif to sleep with the enemy so that they will be free. When she refuses to do so, they try harder and come up with a better plan to manipulate her, and finally, by lying, they succeed to do so. This shows how they, including nuns who are forbidden to do so, behave unethically by manipulating her only for their own benefit. The theme of manipulation is apparent in other cases also, such as Monsieur Loiseau who sells poor quality wine for normal prices, only by lying and cheating his buyers.
Self-sacrifice
As displayed by Boule de Suif, self-sacrifice plays an important role in the story. Boule de Suif is regarded as someone from the lowest social class, dirty and immoral, not worthy of sitting among the rich. Yet is she who gives them food when they have forgotten to take things to eat during the trip. She doesn't have much, but she shares what she has even with those who offend her.
When the Prussian general asks her to sleep with him, Boule de Suif doesn't agree because her moral doesn't allow her to sleep with the enemy, but if she doesn't. she would keep her companions from reaching their destinations. To allow them to continue, she decides to sacrifice her moral and thus she sleeps with the general.
