In stories, as in life, all things must end.
Sometimes, a story wraps everything up – complete with an elegant little bow – and ties off every loose end. This can be emotionally satisfying, and mentally calming. However, it can also feel artificial, or forced, or false; the writer is using a crowbar when a gentle nudge is required. It can also make the ending seem to drag: once the interesting bit has been resolved, the story shouldn’t over-stay its welcome.
A more realistic, and intellectually stimulating ending wraps up the main issue, but leaves a few loose ends. When it works, this (along with beginning a story in medias res) gives the story/setting the sense of perpetuation: it didn’t just spring into existence for the purpose of the story, and it won’t just disappear now the story is over. What can go wrong here is either leaving too many loose ends, which is unsatisfying to the reader, or leaving the wrong kind of loose ends.
To clarify, the wrong kind of details to leave unanswered are those that are important to the resolution of the principle motivating question (i.e. the central point of the story – for example, “Will Frodo succeed in destroying the One Ring?”). Leave enough of these unresolved, and the story will not feel finished.
Oddly enough, a recent movie (not named, and spoken of in vague terms to hopefully avoid spoilers) has been noted as ending badly. Various critics have opined what they think is wrong, but one interesting idea is that, besides not enough being wrapped up, a fairly central question is resolved too well. The hero has lucked out and solved one of their major problems, while leaving others open-ended, thus feeling like an annoying sequel hook and at the same time removing something that made the protagonist interesting.
What got me onto the topic of endings (besides bleakness of mood) was finishing The Silver Chair. It ends rather well (not carrying on too long, dealing with the relevant issues), albeit bittersweetly. The lost prince is returned, just in time to visit his father’s deathbed. Hope is presented for the future, but explicitly deferred.
The other thing I’ve noticed about the book (besides the idea of instructions that we try to follow but continually mess up because we’re not thinking about them at the time they become important) is the amount of influence from mythology – particularly the archetype of venturing into the underworld to bring back a lost or enchanted friend.
I do think it’s a good book, but it’s not really a joyful one. And, as I’ve mentioned earlier, when I was a kid, the whole scene with Prince Rilian strapped to the eponymous chair, begging to be set free, but the children not knowing if he’s currently sane and safe, or bewitched and psychotic. Freaked me out right proper. Not a good chapter to hear (in an emphatically-acted audio book) right before light-out. It’s a lot easier to deal with as an adult, but the concept is still a hairy one.