One major theme in Sense and Sensibility is the contrast between the rational mind and emotions. Jane Austen wrote this novel as a protest against the passionate ideas found in popular works of the Romantic period. The novel teaches that it is far better to rely on sense when it comes to matters of the heart, then it is to be guided by uncontrolled emotions.
Elinor represents the theme of "sense," or the rational self. She is very perceptive and wise, and frequently advises her mother. Even though she experiences the same heartbreak as Marianne with respect to falling in love with Edward who is already engaged, she doesn't abandon herself to her emotions and even manages to keep her sorrows secret because Lucy asks her to.
In contrast, Marianne, who represents the theme of "sensibility" and passionate emotions, falls into a grief so deep that it nearly kills her. When Willoughby becomes engaged to another woman, Marianne walks out in bad weather just to see his house, giving herself a life-threatening fever.
Another important theme is ideal love. Because he is idle, Edward falls for Mr. Pratt's daughter and has to hide his engagement from his mother because Lucy is not wealthy. Edward is later ashamed of his choice, realizing that Elinor is far above Lucy in beauty, sense, and other virtues. He confesses that his attraction for Lucy was a "foolish, idle inclination" (Ch. 49). Similarly, Marianne falls for Willoughby because he is handsome, passionate, vivacious, and shares all of her tastes. But he is not as virtuous as she believes him to be and instead marries a wealthy woman because he is extravagant. Later, Marianne who once believed wholeheartedly in only violently passionate marriages, marries Colonel Brandon based on no feelings more than "strong esteem and lively friendship" (Ch. 50). In other words she marries a man who is a dear friend because she values his kindness, his virtuous mind, and his gallantry.