] Marianne is a beautiful, engaging and extremely attractive young woman. What better way to make sure Marianne does not develop any interest in this worthy, rich, respectable and eligible man that her friends and connections had picked out for her before Willoughby came on the scene than to run him down at every opportunity and encourage Marianne to agree?

Well said. I came to the same conclusion. In a word it's 'jealousy'. Even though Marianne doesn't see it, Willoughby is no fool and knows full well that Colonel Brandon is excellent competition as a suitor for Marianne.