I do think Willoughby is jealous and envious of Brandon for his wealth (as many have said) but also because of the kind of man he is. I think the jealousy and envy of wealth is easy to see in Willoughby’s sneering remarks about nabobs, gold mohrs, and palanquins and complaints that he will not buy his mare and has two new coats a year—it is like listening to a spoiled teenager, overdrawn credit card in hand at the mall.

The jealousy and envy of Brandon as a man is a little more speculative IMO. As far as we know Brandon has done nothing to incur Willoughby’s dislike but I think it is obvious by his snide remarks both in and out of Brandon’s company that he does indeed dislike him and he has no respect for him. Willoughby only pays lip service to respect and IMO is lying when he says he does not dislike Brandon in Chapter 10.

Then just thinking on human nature in general, my only reference observation, is that people are often jealous and envious of those they feel inferior to—instead of working to improve themselves they denigrate the person they feel inferior to. Is Willoughby insecure for some reason? Could being poorer than Brandon make Willoughby insecure? :D

In Chapter 13, Willoughby rudely voices the accusation of Brandon writing the letter himself because he cannot bear a party of pleasure while Brandon is still explaining his refusal of Willoughby’s request to delay his departure six hours. Willoughby’s accusation to Marianne then is a petty revenge for Brandon’s refusal but I also see another component, the accusation of “cannot bear a party of pleasure” is something that Marianne is sure to despise since all young men—

Whatever be his pursuits, his eagerness in them should know no moderation, and leave him no sense of fatigue." (Chapter 9)