Being a writer can be hard. I don’t mean just the time consuming part of thinking up a story, writing it down, flushing out ideas and actions, going back to edit, and getting it out to people. That’s just the obvious stuff.
If you have a story with any sort of length; you have to be the expert on everything about it.
Every sort of fictional story exists in a universe where the events of the story happen. I mean, certain fictional stories; especially historical fictions, could happen in this universe we call ‘reality’. Maybe it did, is doing, or will do; maybe not. But you know, it could. For example, the Star Wars ‘Universe’ takes place a long long time ago in a galaxy far far away. This could be our universe, and we would have known nothing about it had it not been for time/space traveler George Lucas to create the documentary.
Wherever or whenever this story of yours happened, you’re expected to be the all knowing being of it; god. Even if something is not discussed, implied, or even relevant to the story, the author has both the right and the duty to supply any facts or details about it.
For example, J.K. Rowling could say that the Sun in Harry Potter Universe was in fact a copy made by wizards after the original Sun burned out, saving all living things on Earth. Technically it could be plausible because Rowling said so, and doesn’t interfere with any lore considered canon in the books or extended universe. (Maybe. I haven’t read the books in a long time…)
Some things of course are more mundane. When is this random character’s birthday? Oh; she’s a pisces like me. I mean it doesn’t effect the story in any way, but at least I have peace of mind knowing we’re compatible… Fans, am I right?
Which brings me to another facet of writing; the people who read it. In high school, I hated books and reading because the teacher broke them down into mindless critique and analysis. I still never learned the plot of Tale of Two Cities. Authors write; some like Dickens add a bunch of subtle commentary in their words, and others don’t. No, I don’t think Little Red Riding Hood has Communist undertones.
The point I’m trying to make is that people love reading between the lines, even if there is nothing explicit. That’s where fan theories come from; that and wild imaginations creating narratives for characters that aren’t under the scrutiny of the narrator.
Technically it would be under the power of the Author to reject or confirm these. Did background character A and B get together after the story ended? I saw how they looked at each other that one time. Or you know, just ignore them outright because you’ve moved on or they’re just impossible inane.
If you think this stuff is kinda cool, check out the TV Tropes Page I guess.