Inseparability
You know the type—lovers who are so well matched, their lives so intertwined, that you can't talk to one without every word you say sooner or later reaching the other. They're the power couples, but it's more than just the knowledge that they share more than a passing fling together; it's that feeling, like they're not so much two people as just two halves of a whole, like you consider them to be not so much two people as one connection, one bond, one spirit of harmony. They're the characters in books and movies whose every last scene features them in the context of their relationships. They're the forty-something married couples you meet who only go out with their married couple friends, or else who have no choice but to adopt each other's single friends as their own friends as well.
They're Rachel and Finn. And this new development scares the hell out of Kurt.
They dated all summer, but Kurt still can't get used to Finn and Rachel being the latest glee power couple. It's strange enough to sing a duet with Rachel Berry without having to deal with the unavoidable sense that befriending her is just an extension of befriending Rachel-and-Finn. Because he's definitely getting that sense, and given where he and Finn left things this week, that's definitely not something that Kurt is entirely comfortable with.
Somehow, they don't fight at all over the song selection. Rachel's keen on making a mash-up of "Get Happy" and "Happy Days Are Here Again"; she insists upon singing the chorus for the latter, citing Barbra Streisand as her idol since childhood in defense, but Kurt can hardly complain about taking Judy Garland's part. To his surprise, their voices blend beautifully together. To his greater surprise, their rehearsals are... calm, relaxed, if not downright amiable. Maybe it's just him, but Rachel's criticisms seem a lot more constructive and less condescending than usual this time, and she even goes so far as to compliment his vocal quality and range once or twice.
She brings up Finn one mortifying time, to his horror. "Finn told me about why you didn't go through with the duet with Sam," she says, "and I just want to say that I admire you for doing the right thing. I can't even imagine how hard it must have been for you to do that, even to hear Finn out about it. I know how much you cared about him, and it can't have been easy to hear that from him, of all people."
Kurt lowers his eyes and wants this conversation to be over over over. Rachel doesn't bring it up again.
(He and Rachel are more similar than one may think, and maybe Kurt doesn't have to be lonely, but when it comes to Finn, all he wants is to curl up inside his denial and pretend to the world like there's nothing to talk about. Some things are just too hard to face any way other than alone.)