Star Wars: The Last Jedi - 39 Things We Learned From The Director's Commentary
25. Finn And Rose
One of the things Johnson loved was the notion that Finn has left First Order but hasn't really joined Resistance, and instead everything he did was to help Rey and came from his loyalty to her, which is where he starts this movie too. He's trying to save his friend, not help the bigger cause, by removing the beacon to a safe location, which led to pairing him up with Rose who could show him the more important purpose the Resistance was serving.
There was an exhaustive casting process for Rose, who was originally more of a "grumpy Eeyore" type, but Kelly Marie Tran's spirit changed it to being the character we see on screen. Her being a big fan of Finn was a late re-write, in order to reflect Tran's own personality, as initially the character was going to be more sceptical of him.