I don't re-read books with depressing endings, or endings that leave me irritated, or frustrated, or simply unsatisfied. Cliff-hangers are acceptable, if I think the author plans to continue the story. But stories where the good guys gets the shaft, or the wrong people end up together, where injustice triumphs and there's nothing anyone can do about it piss me off. This is why I spend so much time and attention on trying to make sure that my books are neither depressing, nor disappointing. Maybe I'm a slave to the happy ending. I can live with that.
There's more than enough painful injustice to go around out here in the real world, thank you. When I read a book I am looking to escape. I want someone to tell me a bedtime story about people who overcame obstacles and prevailed, to give me a feeling of hope that maybe, just maybe, some of us out here might be able to to the same thing. At least sometimes.
It's not enough to kill the villain, either. Sometimes there isn't a clear villain, and sometimes the so-called heroes are just as bad or worse. There are several books and series where by the time the author is done I, as Twain said, wish that they would all go off and get drowned together.
Game of Thrones for instance. Is there anyone in that series who doesn't need hanging besides the children that they like to murder? The writing is well done. The characters are assholes. In Wheel of Time. I couldn't really force myself to cheer for Satan, but I was sorely tempted. By the end of it I wasn't merely wishing that all three of the supposedly godlike heroes would go get drowned, I was wishing someone would come and drown me.
Even the Harry Potter series, which is one of my personal favorites, fell into that trap. By the last book the only character I gave a damn about was the little elf guy, Dobby. When he bought it I just skimmed the rest of the book. I continued on long enough to see that Granger did, in fact, let her poor self-esteem drive her into an abusive relationship with Weasley (an appropriate name at least). Then I closed the book and stuck it back on the shelf. Moby Dick is supposed to be a classic, but I will never read it again, that's for sure. My main complaint is that the narrator is the sole survivor. What makes him so special? Would it have killed that blasted whale to turn around and give just one more lousy tail slap? Movies and tv can be just as bad.
Granted. Injustice, hypocrisy, cruelty, disloyalty, and plain stupidity happen all the time in real life. I. Don't. Care. I don't like reading about it prevailing.
Arthur Conan Doyle's books are true classics, and I have read every one of them multiple times. Louis L'Amour. Robert E. Howard. Rex Stout, Edgar Rice Burroughs. The immortal Terry Pratchett. The even more immortal Mark Twain. These authors wrote books that I cherish and savor. Why? Because after I read one of them I feel better, not worse. I put the book down feeling relaxed, sometimes even refreshed.
There's a reason that the Robin Hood stories have been passed down the centuries. He is a victim of injustice, but he doesn't sit on his hands and take it. He kicks ass. There is an equally good reason that the King Arthur legend has stood the test of time. King Arthur dies tragically, but for a time at least, honor and justice ruled the land. It shows a higher standard to reach for, and the legend ends on a note of hope.
Not to say that the protagonists in those books are necessarily good people. Many of them are deeply flawed in various ways. A lot of the authors wrote in a time that had a barbaric moral code. But even if their beliefs are politically incorrect by modern standards, all of them had something greater than themselves that they were willing to stand for. And at the end of the book evil is defeated and the backstabbers go down.
One thing worthy of note about fan fiction is that people write fan fiction to fix things. Either they think the author screwed something up, or they think the story isn't done, or they want to know what happened about a detail that was left hanging. In other words, the ending was unsatisfying.
You have to make people close the book wishing that it was just a little bit longer. Otherwise you have failed as a storyteller.