SPOILERS FOR STAR WARS 7: THE FORCE AWAKENS! Obviously.
Alright, so I've seen Episode 7 three times now, and I think I have a theory regarding how Rey ended up on Jakku. This theory has probably been thought of and discussed before, but I haven't found it anywhere, so I'm making a post about it.
In The Force Awakens, Rey, our lead protagonist, has spent years on the desert planet Jakku after being dumped there by someone. We assume that it is a family member she is waiting for, but, she's been there so long, it seems she neither remembers how she got there or who she is waiting for.
Most fans, myself included, assume that Rey is somehow a member of the Skywalker bloodline, whether she is Luke's or Leia's daughter has little effect on my theory, but it is important. (Many fans theorize that she is the granddaughter of Obi Wan, but I personally think that her raw power is too great for her not to be somehow descended from the 'chosen one'. Even if she does end up being Kenobi's kin, my theory would not be affected tremendously.) Neither Luke nor Leia would leave a young child on a desert planet to fend for herself with no parental guardian, even if it was to protect her from Kylo Ren and his knights. Obi Wan at least found Anakin's step-brother to leave Luke with when hiding him.
So, if the Skywalkers didn't leave Rey on Jakku, who did? I think that the answer is revealed in the details found in Rey's lightsaber-induced vision. At one point during this series of distorted imagery, Rey turns around to see a man in armor being impaled by Kylo Ren's lightsaber. I don't think this scene was simply a vision of an outside event. I think that as a child, Rey was actually there, standing in the same spot she stands as an adult in her vision. I believe that the setting of that particular flashback is Luke's temple where he was training young Jedi. I also believe that Rey was attending said training. Finally, I think that the man that Kylo Ren stabs may actually be a member of the Knights of Ren.
Why would Kylo Ren kill one of his own men? I believe that Kylo, aka Ben Solo, knows of his blood relation to Rey (be it cousin or sibling). The film makes it clear that Ben has not fully embraced the dark side. If Snoke had sent him and the Knights to kill the younglings, I don't think Ben would have the strength to let his baby sister/cousin die. Perhaps the man who Kylo Ren stabs was about to kill tiny Rey, and Kylo Ren killed him to save her. Then the question on his mind would no doubt be "What do I do with this kid now?". He couldn't leave her at the temple to be trained by his uncle Luke. That would be aiding the enemy. Obviously, he couldn't take Rey back to Snoke, because that would mean certain death for the child, and a great deal of punishment for Kylo Ren because of his failure to kill all the younglings. So, Ren decided to instead drop the innocent little girl off on a dusty backroad planet where no one, her family or the First Order, would think to look for her. My theory is that Kylo Ren left Rey on Jakku.
Several occurrences in The Force Awakens reenforces my theory. The vision of Kylo Ren stabbing a man (clearly not a youngling, but a man in armor) is a root of the theory. Directly following that imagery, Rey's vision continues and we see 3 or 4 year old Rey on Jakku crying as an unknown ship flies away. The fact that these scenes happen in direct sequence gives the idea that the events are linked. Another reenforcement of my theory is the undeniable fact that Kylo Ren knows who Rey is. Remember the "WHAT GIRL?!" line he has when he learns that BB8 was allowed to escape Jakku? If they were on some other planet, Kylo Ren probably wouldn't have freaked out as much. But Jakku is where he left Rey. He knew there was a good chance that she was still alive and still on the planet. Kylo Ren gives several other indications throughout the film that he knows of Rey's origin. It seems that everyone but Rey, Finn, and Po know exactly where she came from.
So, in summary, I believe that Rey is a member of the Skywalker family, and that Kylo Ren, not yet completely on the dark side, left Rey on Jakku instead of killing her. He did so to spare her and remove her from the battle between his old and new masters. What do you think? Do you have an alternate theory? Am I missing anything? Let's have a real discussion in the comments below!