I think and will stand by the fact that Darcy is unsure of his feelings twords Elizabeth, he likes her and can't help himself to try to be near her but apon approachment he freezes up a bit. I think he asks her to dance because he knows she likes to dance and that will ensure him time with her, once he starts dancing I think that his indiffrence or shortness is simply a case of the tounge ties. Darcy is a gentlemen but he is not above being a little akward around the girl he likes. And note that Elizabeth isn't exactly the kind of match Darcy would benefit from, so that too must be on his mind.