Mathilde thinks back on how Julien almost murdered her in the library and figures that for some reason, this gesture makes him truly worthy of her.
She walks up to Julien the next day and asks him to accompany her into the garden. Once they're alone, she starts talking to him about the state of her heart. She makes sure to talk as much as possible about all the other young men in her life. This tortures Julien.
Finally, Julien tells her that he loves her and that he'll do anything for her.
This destroys Mathilde's interest in him, since she only wanted him in the first place because he paid no attention to her. Now that he's groveling at her feet, she finds him repulsive.