Mrs. de Winter has a classic rags-to-riches transformation in Rebecca. She starts off as a paid companion (a low-class job) and suddenly finds herself as the lady of the house at Manderley. Still, she never quite gets over her roots: she always feels like she's perched between classes, never quite at ease where she finds herself. When she becomes the wealthy mistress of the mansion, she's worried about what both the servants and her newly acquired peers will think of her. In a novel with a lot of class tension, there are very rigid behavior codes and conventions peculiar to each class, which also govern how they are supposed to interact with each other. But wait a second: no one follows these codes! All the major characters drastically break the social conventions they so desperately want others to believe they adhere to. Don't you think?
Maxim chose Mrs. de Winter because she wasn't raised like Rebecca was.
Although he masks it well, Maxim doesn't seem to trust or like anyone in his own social and economic class.