In James's well-versed opinion, giving Remus something to fret over other than the impending full moon (like the fact that he and Sirius seem to think that the whole thing was a lark) was like a civil service. Without a bit of levity or misdirected frustration, he'd have far too much time with his far too hurried thoughts to start plowing through the myriad of ways Moony could go about killing people. Really, it was much better to let James and Sirius worry about that, since Remus had more than his fair share of worries bogging him down.
And the weirdness between Sirius and Remus seemed weird to James. Remus's tone. Sirius's obliviousness. Remus's whole... well, okay, he was mostly just being Remus- more or less. (Maybe a little less than normal.) Sirius's huffing and puffing over that girl. Er, his cousin. Okay, maybe he had a little ground there, but the whole thing was a bit much, even for Sirius. And Remus should be used to Sirius overreacting! It just felt like more of the same, more of that wonky distance and division that had crept in as the war went on.
Lacing his fingers behind his head, James kicked back, sprawling out on the ground to drawl, "See, we've even got a back-up plan. Oh, yeah, it'll be great. We'll sort out a room, find some snacks, Sirius and I will hang out nearby just in case something goes awry, and we'll pull the whole full off without a hitch-- and then the two of you can go back to beating each other up or ignoring each other or whatever the hell else you want to do to put a damper on my delightful beach holiday."