In a prior post, I discuss friends who discover they wish to date. One variation within this trope is the family friends who discover they wish to date.
That is, the best friend of a brother or sister will start to date the brother or sister's sibling.
This is (1) not that unusual; (2) seemingly fraught with emotional angst.
1.People buy products, join churches, and marry based on networking. "You know me" is still the most effective way to get people to do stuff (my friends and neighbors do it too!).
Consider that although Bingley obviously doesn't assume that his sister and Darcy are well-matched, he isn't opposed to the idea either. After all, the world of Austen was a cornucopia of nepotism. People did often marry people related to people that they knew. Jane Austen herself was proposed to by the brother of good friends (who impressively remained good friends with Jane after she turned him down, which tells me that knowledge of personality overrode social pressure even then).
It seems rather odd. After all, families were and are constantly trying to match up siblings and cousins and, if Parent Trap is any indication, parents. So why would it be so upsetting?
Based on the contemporary versions of this plot, the angst seems to arise from the level of knowledge of both parties. Ross is upset about Monica dating Chandler until he realizes that they love each other; this isn't just another case of Chandler waffling over women.
Likewise, in Austen's oeuvre, Captain Harville is supportive but somewhat upset about Benwick marrying again (after being engaged to his sister) since he thought Benwick's emotional connection to his dead sister would last longer. He is objective but still bemused.
That is, the family needs to adjust because they are experiencing a kind of cognitive dissonance: But we never saw you two together! Why now?
In historical fiction, this cognitive dissonance is compounded if there is a social gap.
In real life, Austen lost the young man of her youthful flirtations (who many years later still exhibited a tendre for her) because his family needed him to marry well. Lady Catherine de Bourgh is horrified by the idea of Darcy marrying Lizzy, whose family is supposedly below her own (it isn't). In M/M literature, Richard from the Society of Gentlemen series by K.J. Charles balks at taking his valet as a lover, not, interestingly enough, because he thinks he is too good for the man but because he takes noblesse oblige and his duties of rank seriously.
Apparently, the family friend as lover comes with ready-made conflict. From a writing point of view, it lends itself to a workable narrative arc.
