That relationship is one of the highlights of the movie and here are my two bits. Ray is attracted to the vulnerability in Ren. It mirrors her own, as both are desperate to find new father figures and teachers who will fill their need for approval. Ren found a master in Snoke and Vader is the one he really looks to but none can give him what he needs. Ray wants to find her parents and in them a confirmation of her worth. There is also something to be said about wanting to save/change the bad boy in pain, a common motivation in this kind of relationships, that is perhaps more true of women then men (at least for the bad boy part).
Forced intimacy is also a powerful engine. They've experienced traumatic events together and they have to subsequently share their pain when they're at their lowest. Ren when he's rebuked by Snoke for his failures, Ray amidst her frustration with Luke. Not to mention the whole you killed my father figure type who was your real dad thing.
And possibly, Kylo being more experienced and knowledgeable, not just about the Force, but about the entire universe of characters and events Rey is only just penetrating. That is always a draw.