As the title suggests, the primary theme of Sense and Sensibility is the use of sense vs. sensibility. In protest against the romantic literature of her day that praised extreme emotionalism and focused on the needs and wants of self above the common good or community, Jane Austen used Sense and Sensibility as a warning to show just how dangerous violent, uncontrolled emotions, or sensibilities, really are. Hence, Austen juxtaposes two sisters with two different philosophies.
The eldest sister, Elinor, governs all of her choices and actions with sense and even believes in controlling her emotions. Her sensibleness and her ability to think calmly and coolly make her an indispensable counselor for their mother. In contrast, while Marianne also has sense and cleverness, she prefers to be governed by her passionate emotions. As Austen describes, Marianne is "eager in everything: her sorrows, her joys, could have no moderation" (Ch. 1). Even Mrs. Dashwood has a tendency to think and act like Marianne, and both encourage each other to be passionate about their emotions. Hence, when during the course of the story, both sisters become equally brokenhearted due to love, Elinor makes the conscious decision to govern her emotions while Marianne violently gives in to them, leading to a dangerous fever that nearly takes her life.
Elinor argues that it was her duty to keep her emotions in control. For one thing, she made a promise to Lucy to continue to keep her engagement to Edward a secret. Not only that, Elinor knew just how much hearing of Edward's engagement would upset Marianne and her mother; therefore, she was eager to protect them by keeping them from finding out. Marianne, on the other hand, feels it's her right to express her grief, even at the cost of hurting other people. However, she soon understands that her philosophy about unguarded emotion was wrong. She even understands that her behavior had been selfish and had nearly cost her her life, as we see in her lines:
I saw that my own feelings had almost led me to the grave. My illness, I well knew, had been entirely brought on by myself by such negligence of my own health, as I had felt even at the time to be wrong. (Ch. 46)
Hence, we see that Austen is showing us that all things must be governed by sense, even one's sensibilities, or emotions, making sense vs. sensibility the main theme.