Why does Violetta tell Alfredo that she loves Baron Douphol?

In Giuseppe Verdi's La traviata, Violetta tells Alfredo that she loves Baron Douphol to fulfill a painful, self-sacrificing promise she made to Alfredo’s father, Giorgio Germont.1

To protect the reputation of Germont’s family and ensure his daughter can marry a respectable man, Germont convinces Violetta that she must leave Alfredo forever. Because Alfredo would not believe her if she simply claimed she no longer loved him, and because he would likely follow her if she left without explanation, Violetta feels forced to invent a new attachment to the Baron to drive Alfredo away. By claiming she loves the Baron, she successfully pushes Alfredo to abandon her, though the deception causes her immense emotional distress and leads to a public confrontation at Flora’s ball.1