At Lady Catherine de Bourgh's house, what is the explanation Elizabeth and Mr. Darcy both believe explains his awkwardness around strangers and her...

Elizabeth essentially takes responsibility for her own mediocre piano playing by saying she doesn't play the instrument as well as she would like, but believes this is because she does not take the time to practice as often as she should.  Elizabeth claims the same unwillingness to practice lies at the root of Mr. Darcy's awkwardness in social situations: if he would only take the time to put forth the effort and practice more often -- basically, if he attempted, even a little bit, to speak to acquaintances, dance at balls, enjoy himself in the company of those he does not know well, and the like -- then he would begin to act and feel a great deal less awkward, just as more practice would improve Elizabeth's skills at the piano.