I feel in this debate that it is a matter Darcy has been thinking much of and that he is almost criticising or preaching to Bingley about the dangers of being too easily swayed. Bingley takes it very good-naturedly.

I can see the validity of both Darcy and Elizabeth's point. While it is good not to be hard-headed over a matter which does not deserve too severe firmness (and it is 'sweet' to please one's friends), it is also not good to be too easily swayed by friends who may lead you wrong - which is I think Darcy's point.