To begin with, Luke's devotion to the force became apparent at the end of Return of the Jedi, when he saw Obi wan, Yoda and Anakin after cremating his father, where he remained alone rather than celebrating. Why would Luke have decided to have a child during this time and leave it on Jakku of all places. Whilst that isn't a great reason, here are some others relating to why Rey could be Han and Leia's daughter:
1. Kylo Renn's tone when questioning Hux after he told him there was a girl helping BB-8 escape Jakku and his slight overreaction afterwards pretty much shows he has some kind of link between him and rey.
2. Rey was able to resist Kylo's torture. After trying to read her mind (and failing) she stated "You're afraid... that you'll never be as powerful as Darth Vader." after he said he could see her fears. Renn is the first force user I am aware of that can read a persons mind, and Rey was able to resist it almost immediately. Perhaps such a power is shared between them as brother and sister as an evolving force user should not be able to resist such a unique ability without experience in resisting the ability previously.
3. Kylo Renn mentioned to Rey that she saw Han Solo as the father she never had. Perhaps that wasn't a way of manipulating her emotions but giving her a hint that they are related.
4. Renn took off his helmet for Rey, to show he wasn't a monster. Someone willing to kill their own father would not take off their intimidating mask because a girl asked them to. He obviously knows something Rey doesn't know and wants to reveal his real self to her, perhaps giving her another hint about them being related.
5. Has anyone else noticed Rey and Kylo Renn look very similar? Might not just be a coincidence