Here is my personal fan theory about why Rey is the REAL Chosen One, the one who will unite the Light and Dark sides of the Force. It's a bit long, so bear with me.
Everything below contains MAJOR spoilers for all of the Star Wars movies, especially Episode VII The Force Awakens!
The Theory: Rey is Luke Skywalker's daughter, and thus the REAL Chosen One, who will unite the Light and Dark Sides of the Force, by successfully bringing her cousin Kylo Ren from the Dark Side to the Light, in order to finally vanquish evil in the galaxy
The Original Chosen One: Anakin Skywalker
Anakin transforms into Darth Vader, forsaking his claim as the Chosen One
Anakin/Vader's son, Luke Skywalker, becomes the next plausible option for the Chosen One
Despite this, Luke trains with Yoda, and becomes masterful in the ways of the Jedi.
Luke, however, plans to bring his father back to the Light Side of the Force rather than killing him, which would complete the task of the Chosen One of bringing balance to the Force, through the redemption of Darth Vader.
Cut to Star Wars Episode VII: The Force Awakens.
Luke has been training a new class of Jedi, when one of his own, his nephew Ben Solo, decides he wants to join the Dark Side, and kills the other Jedi Luke has been training, becoming Kylo Ren.