My copy of Etiquette for Ladies and Gentlemen Or, the Principles of True Politeness is a small and sadly anonymous book, published in Halifax ,Yorkshire in 1825, but it does have something relevant to say about the positive attitude required of a gentleman in a ball room.

Let me quote for you, from the Chapter entitled "Ballroom" in the Section "Etiquette for Gentlemen:

At a ball it is not necesscary to display the science and agility of an artist: it is simply sufficient that you dance with ease and grace, and that you enter into the spirit of it as becomes a gentleman.

No where has a kindly and pleasing disposition a better opportunity of showing itself than in a ball-room, no where perhaps is there so good an opportunity of conferring those small benefits and pleasing attractions so gratifying to all.

Remember that a ballroom is a school of politeness,and therefore let your whole conduct be influenced by that strict regard to etiquette such a place requires

Those who are proficient in dancing, and are conversant with the various figures, may instruct the unskilful, and that too in a manner that cannot give the slightest offence, or assume the appearance of dictation.

Carefully avoid all peculiarities in your style of dancing, as well as all attempts to show off; such attempts generally interiere with the pleasure of a dancing party, and the person guilty of them is never long acceptable.
Pages 32-33

Darcy was remiss: he did not enter into the spirit of it all,as reccommended here.

By that I mean he did not dance as much as he ought.

The problem for Darcy is that he knew men were scarce at the Meryton Assembly: they were in great demand as partners. He could clealry see Elizabeth and Mary sitting( and no doubt among other ladies)on teh sidelines waiting for partners. Ladies could not ask to dance: thiri power was only in refusal.The onus socially was on him,IMHO.

This is how these events worked,and was their whole purpose,after all. Young eligable gentlemen met and danced with lyoung eligible ladies. Social interaction was the goal.(Of course some assemblies put strict rules in place to limit some classes social aspirations but in Meryton it apper they did not. Here is an old post I wrote on this subject from the L+T Board.

Darcy had no true excuse not to participate fully.He was not old and interested in cards. He was not married,standing by talking with the married men. His big solicism was realising men were scarce and not acting appropriately- dancing as much as possible, as Bingley did. If there had not been a scarcity of men,there would have been less pressure socially on him to dance. Is this is the only time Ja mentions that there is a lack of avaiable" introudced",men to dance, at a public assembly.? I think it may be.