] For some reason (dense, I'm sure), I've never noticed in reading, but I should have known that a person who agrees to like everything you like is suspicious.

Though I'm inclined to agree that Willoughby was at least partly turning on the charm to win Marianne over, there is a less negative explanation for it too. You know how it is when you've first going out with someone, or even made a new friend you've really "clicked" with. You truly *believe* that the two of you feel the same way about everything, and that just reinforces your excellent judgement in getting together in the first place(!). It's only in retrospect that you realize that a lot of that "twins separated at birth" feeling was a pleasant and enjoyable bit of self-delusion!