Finn didn't start out as a hero. He wanted to get away from the first order. Which no one could blame him for.
Like Poe already said. He needed a pilot. They both needed each other. So they escaped together. And they got separated. Then he found someone who changed his life. Rey.
But even after everything they'd been through he was still ready to get on a ship and leave. Her only being taken made him want to run right back to the people he was trying to get away from. He told Han he didn't know how to disable the shields on Starkiller Base. He only came to get Rey.
There's no point denying it. He loves her. It's obvious. She's been his main reason for fighting.
But in the next movie Rey is gone. She's with Luke, hopefully training. So where does that leave Finn after he recovers? Does he want to continue to fight for the resistance against the first order?
I think TLJ will be good for his character. Love Finnrey. But without her around Finn really has to decide who he is and what role he wants to play in stopping this war.