Little Women by Louisa May Alcott
At the end of Little Women, Laurie randomly marries Amy rather than Jo, who rejected him. Also, Beth is dead.
I'm about to make a claim for which I have no evidence (and zero desire to look into the matter further), but I think this is the first time people ever got THIS MAD about a fictional couple. Part of me is like "Okay, I get it, real life is messy and people don’t end up together for all kinds of reasons, and if Jo didn’t love Laurie she would have been doing him a disservice to pretend otherwise."
But the other, louder, and altogether more compelling part of me is like "I DID NOT READ 465 PAGES TO WATCH LAURIE END UP WITH AMY. IT KIND OF FEELS LIKE YOU WERE JUST THROWING PEOPLE TOGETHER AT THIS POINT, LOUISA MAY ALCOTT. STOP TRYING TO MAKE ME CARE ABOUT PROFESSOR BHAER, I BARELY KNOW WHO THAT IS."