Darcy knows that he is extremely eligible, and I think he is used to thinking that any woman would jump for joy at the thought that he was interested in her (take Caroline Bingley as an example!). As a man of principle, he knows that it's wrong to "waste the time" of a respectable young lady bent on marriage by making her think he has serious intentions when he really hasn't, so he is being extra-careful (IMO a bit ludicrously so) not to given Elizabeth any encouragement.