So Finn is basically amazing as a spec-ops type of guy for the Resistance and he gets sent into a highly dangerous situation to save another high profile pilot, maybe one who was a big shot in the Republic before it got blown up. Finn comes back, all successful from the mission with the pilot in tow, only for Poe to discover the dude was basically his BIGGEST RIVAL back when he was in the Republic, and even worse, his arch-nemesis is basically half-in-love/lust with Finn for being his savior.

Poe is jealous and for the first time, lacking self-confidence, because you know, while Poe shared a daring escape with Finn as well, they got shot down and Poe crashed their TIE. Meanwhile, Rival guy got Finn back in one piece in a couple of hours no worse for the wear, and also seems to have no qualms about trying to woo Finn.