The are so relaxed because they just fucked each others brains out.
I’ve seen some comments that some people are disappointed that they didn’t show them in bed like Quinn and Santana and, sure, that would have been great, but I really like this. Seeing them talking while Kurt is dressing, Blaine coming over, helping him put his jacket back on, standing together looking in the mirror. This whole scene is very intimate and beautiful to me. Also, I love the way it was shot.