Jess had said that she didn't like Finn. To him, that was fair and understandable. He had been a Stormtrooper all his life, part of the machine that killed trillions- He feels the weight of that burden and gets her anger.

But this. This makes Finn not like her very much in turn. She might be important to Poe but that attitude wasn't acceptable.

He turns serious, scowling, milkshakes and entire diner forgotten. Jess could hold on to her grudges, whatever, but she wasn't about to do this to Poe. "There are no sides here," he repeats, firmer. "Only people. Both sides were only ever made up of people. Most of them are trying to move on."

Jess was going to be a major problem with the truce, he could already see it.