The idea of being pissed off about not keeping a promise made in prison set off all sorts of tiny little alarm bells in Sirius' head. At least he had eventually made some sort of truce, but how thick was Potter to think someone in prison would say the same things once they were free? And, well, it didn't speak well of the blooming relationship, really.
"Well, at least you're still in," Sirius said. "Just, don't let him try to talk you out of it if that's not what you want, yeah? There's plenty of legit reasons to opt out, but don't let him pressure you into anything."