Poe is nice to everyone, Finn knows that. But no one is he kinder to and more gentle with than his little droid, BB-8. Finn finds that seeing Poe interacting so gently with BB-8 inspires feelings in his chest not unlike what most people feel when they find out their crush is good with kids: he's swooning.

Bonus if Poe is oblivious, but BB-8 knows and purposely does things to make Poe fuss and play with him more in front of Finn.