I've often wondered WHEN Darcy decided to ask for Lizzy's hand. Before he left Rosings? On the way over? When he sees her at Hunsford and is overwhelmed? I don't think there is evidence, so we can only speculate. I like to think it's the last option.

The proposal is both in character and out of character at the same time. It's self-centered nature ('of course, she'll accept me, the great Mr. Darcy') is consistent with the Darcy we've seen up to this point. On the other hand, the decison to propose seems to have been made when Elizabeth didn't show up at dinner -- an impulsive action unlike anything we've seen from the deliberative Mr. Darcy. Her absense shows him how much he is attracted to her, feelings that can no longer be quashed by reason. On the way over, he may still believe that he just needs to see her, but when he actually does, it is not enough, and emotion takes over.

The combination of impulse and ego yields a proposal that is doomed to failure. Without ego, his impulsiveness could have translated into a clearer expression of his ardent admiration. With more advance consideration, he might have have admitted the need to "sell" himself more (although we might have wound up with something more like Mr. Collins who enumerates the advantages of the match before he is run away by his feelings -- perish the thought.)