Reason and emotion as survival tools
Elinor and Marianne don't represent opposites so much as two different strategies for surviving a world that gives women very little control. The novel tests both strategies and finds that each one alone is insufficient. Elinor's sense protects her but isolates her. Marianne's sensibility connects her but leaves her exposed.
Economic dependence shapes every choice
The Dashwood women have no financial security of their own. This isn't background detail. It determines who they can afford to love, what risks they can take, and how much they can say out loud. Willoughby chooses money over Marianne. Edward is disinherited for honoring a promise. Money moves every plot point.
Social performance and private feeling
Characters constantly perform emotions they don't feel and hide emotions they do. Elinor smiles at Lucy Steele while in pain. Willoughby performs passion he doesn't fully mean. Mrs. Jennings gossips but also genuinely cares. The gap between public behavior and private reality is where most of the novel's tension lives.
The danger of romantic idealism
Marianne builds Willoughby into an ideal before she really knows him. Her love is partly for the man she imagined. When the real Willoughby appears, the collapse is total. Austen is not saying don't feel. She's saying that projecting an ideal onto a real person is its own kind of blindness.
Loyalty and patience as forms of love
Colonel Brandon waits. He doesn't perform. He acts quietly when it matters, like arranging a living for Edward when Edward has nothing. Austen builds a case across the novel that this kind of love is more reliable than Willoughby's intensity. Brandon's love is proven by what he does, not what he says.