They need to work together to survive. Maybe they're stranded in the wilderness somewhere on an unfamiliar planet. Bonus points if Kylo is injured but hides it to avoid showing weakness. At some point the facade crumbles and Rey realizes what's up. He expects her to mock him but she ends up caring for him and touch-starved Kylo doesn't really know how to deal with her kindness/concern. They start to get closer despite their misgivings and the attraction only gets worse until they just can't fight it anymore.

Super bonus points if they're forced to cuddle to share body heat at some point. That is one of my favorite tropes.