Darcy is astonished by Elizabeth's rejection, having convinced himself that she would welcome his proposal.  Not only does she refuse, but she insists that she is offended by his manner of proposal and by his behavior towards people of his acquaintance.  Elizabeth cites two specific grievances.  The first is Darcy's interference in Jane and Bingley's relationship.  The second is the interaction between Darcy and Wickham.  Darcy feels he needs to defend himself, particularly in regards to Wickham.  Part of his desire to defend himself must be to ensure that Elizabeth, whom he really does love, doesn't get taken in by the lies of Mr. Wickham.  Regardless, Darcy feels very strongly that he must defend himself against charges that are without truth.