Is it possible that Charlotte missed an opportunity in the past of "fixing" a man and is now sorry she was so reserved towards him? It certainly would explain her present feelings about showing more affection than one feels, in order to give the man no doubt of her affection.

Someone asked earlier why she interrupted Lizzy talking to Mr. Darcy and forced her to play the piano at Lucas Lodge, and this possible past of hers would explain that too. She and Elizabeth have been talking about Mr. Darcy's observing Lizzy often, and it's probably occurred to Charlotte that he at least admires her looks. Charlotte is a good friend to Lizzy and seems to want to put her in a good light so she can have a chance at happiness. And performing for everyone at Lucas Lodge would accomplish this.