A criticism of stories we often see is something in the vein of “this is too unlikely.” The reader finds the story ridiculous, outlandish, contrived, unrealistic, impossible, or other things the thesaurus might suggest that mean that the reader doesn’t think such events could occur, even if the story itself is fantasy or science fiction or otherwise involving things that aren’t real in the here and now.
It’s very easy for a writer to find him/herself in the position of writing about things that seem less likely than hitting the Powerball grand prize every week for a month. Setting traditional genre divisions aside, writing about unusual events is a major genre and we’ve all read those stories. A hobbit just happens to stumble upon an ancient ring of power in a dark cave, that sort of thing. It’s a valid storytelling choice; you’re not wrong as a writer if you write about an event that really would be about as likely as being run down by a rampaging zebra in the middle of Siberia.
The trick is, of course, suspension of disbelief. If you’re going to write about stuff like that, you need to do it in a way that invites the reader to look away from the unlikely place because they enjoy the story more than they’d enjoy pointing and saying, “look at how silly this really is!” Consider that Bilbo finding the ring in that dark cave was as unlikely as that zebra showing up in Siberia right when and where you are. Consider that people have pointed out how unlikely that was. And consider how few people really give a damn even as they acknowledge that yes, it really was a ridiculously huge coincidence if you think about it. Saying ‘the ring is a powerful magical artifact and wanted to be found’ is an awfully thin veil for it, especially if you consider that it was already as mobile as it wanted to be with Gollum, if it had such wanting-to-be-found influence.
But we, as readers, don’t really care. Because it’s a damn good story. Tell a damn good story and you can get away with making it an unlikely one… as long as you don’t overdo it. Even Tolkien couldn’t have gotten away with putting a coincidence like the one we’re talking about in every chapter.