I actually tried to post this last night but I went to Preview and when I flipped back, I'd lost my text. I was too tired and disgusted to repost as I ususally do. It was originally under Kathleen's post directly below this because I wanted her to know that I really appreciated her '...he would be well-advised to criticize Brandon likewise...' line. :)

A slightly different post but but in essentials, the same:

I like the jealousy/envy theories in this thread which is the answer I came up with when I posed the question of what is Willoughby's problem. But then I wondered why because from where Brandon is sitting, Willoughby seems to be sitting in the winner's seat.

Willoughby is younger by far, extraordinarily handsome apparently (manly beauty, whoa!), his property might not be as great as Brandon's but it sounds decent enough and he stands to inherit Allenham which ought to go a long way towards closing the gap, and while he hasn't earned the respect that Brandon has, he doesn't seem to lack respect either. Brandon is not even trying to compete with him. In fact, it doesn't seem Brandon ever felt he had any chance and has never considered himself in the running. It is obvious that Willoughby already has Marianne's undying adoration and all thoughts of Brandon as her lover by others has been transferred to himself.

I think Robin's thoughts of Willoughby's envy of Brandon's financial situation is a good explanation. When Elinor is trying to sort out the engagement situation between Willoughby and Marianne, she notes that marriage between them would have to wait because of Willouhby's lack of money to support himself in the style he likes along with a wife and family. If Willoughby has already proposed or plans to, I can see him being resentful that Brandon is in a better position than him and would never have to ask his bride-to-be to wait as he has already had to do or will have to. But it's not an excuse; as someone has pointed out, Willoughby could have been living more frugally and managing his money better instead of envying those who have.