I think that bit about knowing how it's going to end is what matters. I'm a friend of the fluff, I get scared for characters quickly and feel bad for them when they suffer in someone's fic, but often I get similar ("NOOOO, omg, why is this happening STAHP D:") reactions from readers to my own fic, where I think 'What? This isn't so bad, is it?' so I think it's true that you can be a lot more relaxed about the stuff you personally control. I think that lack of control (I can't do much at all as a reader, the author controls everything) is what makes bad stuff happening in a fic so terrifying.