] All these are good reasons for Willoughby to jealous of the Colonel. But, I think that he also expresses dislike because of Marianne's attitude. We are told in Chapter 10 that W conforms his opinions to hers.

Absolutely, Willoughby conforms (which is the operative word here)himself to Mairanne's opinions and thoughts. IMHO if he did have an opposing thought, he'd keep it to himself. I do believe they have much in commom but by the same token I believe that Willoughby has figured out Marianne's penchant for the romantic, the passionate and having similar minds with whoever she plans to be in love with and he knows how to make himself fit the mold. He manages very well to keep himself the apple of Marianne's eye while making sure the Colonel is relegated to nothing but a core which we all know is tossed away.

] Hearing that Marianne holds disdain for Colonel Brandon's age, lack of liveliness, etc., encourages his natural jealousy.

] It makes me wonder who Willoughby really is.

Indeed! :-)