The Last Jedi is coming out in December and I watched a behind-the-scenes video that basically confirmed the theory I had for a while. (The video is on the official Star Wars YouTube channel.)
Anyway, the theory is that Kylo Ren is Luke’s son, not Han and Leia’s. I mean, would George Lucas make it so obvious? Han and Leia’s son, Jacen, who later becomes Darth Caedus ( I think that’s spelled right, comment Fizzpop if it’s not), kind of enticed Ben with the Dark Side, and as Luke slowly became more distant, it grew in him.
And if you’re thinking, “why do Han and Leia call him son” it is probably because he started to look to them after Luke grows distant, and they think of him and care for him as a son.
This is a theory that might actually be right, I don’t know.
Comment Stooky if you agree with this theory!