You can find the gist of it at the start of the story when it says
"it is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife."
The universal description of marriage is that it is a social necessity which has to be done above one's own social stratum in order for the family to increase their financial and social standing. Hence, it may not necessarily be done for love, but for prestige and ulterior motifs aside from love.
However, in Pride and Prejudice love is supplanted by the need for prestige in many occasions, and pride interferes in the natural flow of emotions of some of the couples.
In fact, in Pride and Prejudice we find how all marriages made for appeareance, prestige, or social standing inevitably end unhappily. Those are the elements of society that Austen detested the most and the ones she took a dig at with her novels.
For your aid, I am giving a couple of links. These will help to get more information in deail.