I was overhearing my dad talking some time back, and he was complaining about books that don’t resolve everything in the end, as if they were always setting up for a sequel. However, as a writer, and somebody who understands fiction, I disagree. Endings don’t necessarily have to resolve everything, sequel planned or otherwise.
That being said, I feel I should at least resolve the main plot, and it should be satisfying. I can’t just leave every plot thread dangling.
But it’s not always about a sequel. I hear of some writers who don’t write sequels because they want to leave everything to the reader’s imagination. I personally like sequels, as there’s only so much you can explore of a world in just one book, but that’s beside the point.