Any story reflects the mood of its period. The marriages of Darcy and Bingley are symptomatic of the mood of the period.
Pride & Prejudice was written at a time of violent social revolution in France, where the old aristocracy refused to acknowledge or accommodate the growing aspirations and power of an emerging middle class. While France resorted to violent revolution, English society chose the evolutionary path.
This evolutionary movement was reflected in subtle changes affecting the way people think, feel and act as well as how the collective responds to their individual actions. The characters and events in Pride & Prejudice depict this evolutionary process in English society.
Her 'mean understanding, little information and uncertain temper' notwithstanding, Mrs. Bennet personifies the aspiration in the atmosphere.
For the Bennet girls, with their estate entailed away, marriage holds out the promise of a better life. Their father, in his quiet, seemingly indifferent manner, wishes for it. All of Longbourn and the neighborhood is on the lookout for eligible men, prospective romance and marriage.
Society is evolving, class distinctions are becoming fainter. The business class, with its increasing wealth moves up the social heirarchy. The energy and bold initiative that is the need of the hour is not being generated sufficiently by the landed aristocracy. The rigidity of the social structure begins to crumble. The upper classes need the vitality and freshness of the lower to breathe life into it. The lower classes are conscious enough to rise higher.
This theme of evolutionary social development is expressed in the story through the institution of marriage. Marriage is an instrument for social evolution in this society because the society is still physical.
Bingley arrives at Netherfield with Darcy at this moment, the right time, as an answer to Mrs. Bennet's, and everyone else's aspiration.