Throughout most of the novel, the unnamed narrator lives fearfully and timidly in the shadow of the dead Rebecca. Mrs. Danvers constantly describes Rebecca as a beautiful, confident, and vibrant woman whom Maxim adored, a woman who was nothing like the insecure narrator. The new Mrs. de Winter can't, therefore, believe that Maxim truly loves her. She thinks she is a sorry substitute for Rebecca and that Max's heart must still be with his first wife. Under these conditions, it is hard for her to truly trust and love Maxim. There is an emotional distance between them that she can't understand, and Maxim's thoughts often seem far away.
After she hears from Maxim the story of how he killed Rebecca—who was dying anyway from terminal cancer—and learns from him that she was a horrible woman who made him miserable, the new Mrs. de Winter finally realizes that Maxim loves her and needs her. This changes everything. As they talk, the narrator recognizes that Max didn't think she loved him: he thought he was too old for her and that she was bored with him because she talked more to Frank than to him. Max, in turn, suddenly realizes that his new wife thought his heart and mind were with Rebecca, which is the opposite of the truth. In chapter 20, they both realize that they have been miscommunicating and that they love each other dearly. Maxim says to her early in this chapter,
I love you so much...so much.
The narrator becomes a new person as a result of what she learns in chapter 20. Textual evidence of this also appears in chapter 21, early in the chapter (page 285 in my edition):
I would never be a child again. It would not be I, I, I any longer, it would be we, it would us....Our happiness had not come too late...I would fight for Maxim. I would lie and perjure and swear. I would blaspheme and pray...Rebecca had lost.
Maxim's new wife becomes brave because she understands he loves her. She is determined to stand by her man no matter what, throwing morality to the winds.
To a modern, twenty-first-century audience, the new Mrs. de Winter might be called into question for continuing to cling to and define herself through a man. We need to keep in mind as readers, too, that all she has to go on is Max's version of the story of what happened with Rebecca. He may be a good person, but people almost always rearrange stories to show themselves in the best possible light and to justify themselves. It might be healthier for Mrs. de Winter to ask more questions and also to try to gain a sense of self that is not entirely dependent on the approval of a man and the triumph of winning out over a female rival for the man.