Some people are able to watch a movie or TV show or read a book and simply let the work exist as it stands. Others, on the other hand, are not content to let the author or director have all the fun. In every story you consume, there's often more than what's going on at the surface level. Sometimes, the creators reveal a deeper meaning that you may have missed the first time. Other times, though, it takes fans and their incredible brains to concoct a theory that is so believable it may as well be canon.
A recent AskReddit thread gave people the opportunity to share their own fan theories that they accept as 100 percent truth. After reading through them, I think you'll find yourself convinced on a number of fronts. Like the one about Willy Wonka and the Chocolate Factory. I did not expect to fall for that one, but I now believe it with every fiber of my being.