Mr Bennett admits to having married Mrs Bennett for her beauty and, because of his shallowness, is punished with a silly wife. It is assumed that Mrs Bennett married for the convenience of money and property (not knowing that the estate would be entailed away due to her failure to give Mr Bennett a son). Similarly, Wickham married Lydia for money (eventually, when Darcy fronts up, pays and forces him) and Lydia shallowly chooses Wickham for his charm and beauty.
