Spirited, passionate Marianne once believed that a person should only love once in life, wholeheartedly, but circumstances teach her that the world does not conform to that ideal. The love of her life was to have been Willoughby, but he jilted her; she scorned the idea of Brandon as a suitor because she knew he had loved someone before. Her marriage to Brandon is thus a sort of compromise, happy enough, but not full of the passion she once believed in.
Marianne must accept the fact that in the real world, practicality and common sense work much better than intense feeling and emotion. In marrying Brandon, she does conform, but it is sad to see her idealistic, feisty spirit "tamed", disillusioned and restrained.