I very much agree that self-interest makes Willoughby’s deceit unlike Elinor’s “task of telling lies when politeness required it” (Ch. 21) which is an act of civility for the benefit of others. Elinor gains nothing personally by speaking more warmly of Lady Middleton than she feels. (:D)

"Elinor," cried Marianne, "is this fair? is this just? are my ideas so scanty? But I see what you mean. I have been too much at my ease, too happy, too frank. I have erred against every common-place notion of decorum! I have been open and sincere where I ought to have been reserved, spiritless, dull, and deceitful. Had I talked only of the weather and the roads, and had I spoken only once in ten minutes, this reproach would have been spared." (Ch. 10)

Marianne is all unchecked enthusiasm and rapturous delight during their first visit (Ch. 10) after the Great Tumble and though Willoughby’s “person and air were equal to what her fancy had ever drawn for the hero of a favourite story” (Ch. 9) she did not approve everything he says. When they differed she argued. They disagreed about books, “her favourite authors were brought forward” and later Elinor teases Marianne about ascertaining “of his admiring Pope no more than is proper” and “his estimating their beauties [Cowper & Scott] as he ought”. Elinor’s comments suggest Marianne marshaled his opinions into (her idea of) reasonableness but IMO it was not due to the justice of her arguments. Rather it was “the force [the rapturous manner] of her arguments” and “the brightness of her eyes” which is a physical manifestation of her enthusiasm.

As Willoughby is not convinced by argument I think he did agree with Marianne simply to gain her approval. Early in his visit Willoughby declares “of music and dancing he was passionately fond” and in response Marianne “gave him such a look of approbation as secured the largest share of his discourse to herself for the rest of his stay.” To me this suggests Willoughby is taken with Marianne’s obvious approval but also that he may quite enjoy it—a feeling on its own merit very reasonable but using deceit to secure Marianne’s continued approval IMHO is not so reasonable. Does it matter if his deceit is about inconsequential or major outlooks on life if it is done only for self-interest? It is plainly wrong for Willoughby to obtain Marianne’s good opinion and eventual affection by deceiving her about his beliefs.

I can see someone softening a stance because they like their opponent but Willoughby goes further than that. He capitulates completely to Marianne on every subject they discuss and that is suspicious in itself. I feel the narrator slyly casts doubt on Willoughby’s veracity by rationalizing his agreement. In saying “any [sensible] young man of five-and-twenty” would “become an immediate convert to the excellence of such works, however disregarded before” suggests it is reasonable to superficially adopt an opinion because others would do the same. How can the frequency of a behavior make it right? If Willoughby’s deceit is reasonable then why offer justification? Is it because there is less credible or agreeable causes? Willoughby is also described as “exactly formed to engage Marianne's heart” and the word “formed” suggests that Willoughby is a designed rather than a natural phenomenon.