I was thinking about this stuff this morning. This isn't a thesis or anything, mostly just observations and wondering. Spoilers through the end of Season 3, mild speculation for Season 4. (I know nothing about Season 4, and don't tell me.)
Also, it's not particularly Rachel-friendly, although I don't think it bashes her, either. I'm just not sure right now how beneficial the Kurt-Rachel friendship is to Kurt right now.
- Kurt and Mercedes were friends when they came into Glee Club.
- Their big commonalities in Season 1 were love of singing, love of fashion, feeling like outcasts, visible minority status, and unrequited affections for people who cared for them but couldn't love them romantically.
- They drifted apart in Season 2, starting (I think) in Grilled Cheesus, when Mercedes unintentionally did all the wrong things in trying to help Kurt to deal with his father's coma. That is, she tried to give her solution to problems to Kurt, which was faith in God, and failed to see that by doing so, she was adding more stress to his life and actually making it harder for Kurt to deal with his pain.
- Kurt ostensibly decided to see her intentions and not her actions, and ostensibly forgave her, but things were not the same.
- Kurt felt even lonelier than he had before, because he no longer felt he could trust Mercedes to accept him as he is.
- This contributes to Kurt's depression and lack of support system (as does his dad's illness, Karofsky, Will and the boys being douchebags).
- Kurt meets Blaine and sees a place where gay kids are accepted on the level that straight kids are. Blaine accepts Kurt for who he is.
- Kurt's resentments against Mercedes bubble to the surface and he's a bit of a douchebag to her.
- Still, Kurt feels grateful for their friendship and feels like he needs to find her a replacement before he dumps her. He tries to set her up with random (cute) black jock. Mercedes wonders why Kurt picked someone black. I have thoughts about what's going on in Kurt's head here, but not sure I can put them into words yet. They have something to do with his feelings of solidarity with Blaine, and like he's found someone who understands, and all of that going way beyond the romantic part. So he assumes that Mercedes needs someone black to get that same stuff from. Also, it might unconsciously be a little of, "You've tried to fit me into a box; I'm going to put you into one, too."
- Their friendship pretty much goes on hiatus.
- Mercedes is replaced by Rachel, who Kurt has reason to believe he has more in common with. Ambition, love of showtunes and Barbra, New York, gay culture, crushes on Blaine, yadda yadda yadda.
- I have to admit, I'm having difficulty right now understanding the emotional underpinnings (that is, the affection part) of Kurt and Rachel's friendship, so I won't be able to go into that in detail.
- Rachel keeps insisting to Kurt that he is her best friend. It comes up a lot in the show, but I don't remember Kurt saying the same about Rachel as often. Does he?
- Why does Rachel want to see herself as best friends with Kurt? Is it guilt over her trying to date Blaine? Is it an unconscious desire to control Kurt? Is it because she doesn't have a lot in common with Finn and wants a male peer who she does share a lot of tastes and interests with? Is it because Kurt reminds her of her dads, and she wants someone like them to share her future with, because she's not ready to let them go?
- Rachel betrays Kurt repeatedly: pursuing Blaine, laughing at Kurt's Romeo, running against him for class president. At first, Kurt seems to accept this as normal Rachel behavior and not something that needs forgiveness. When the class president thing happens, she's gone too far. She has to actively repent, and he forgives her.
- Kurt and Rachel share a dream and pursue it together. At times, they see each other's successes as their own successes, because they are a team and want to get to New York together.
- Kurt no longer complains about Rachel automatically getting all the solos. (Hmmm ... is he under her thrall?)
- Rachel self-sabotages her NYADA audition and almost (subconsciously?) sabotages Kurt's.
- Meanwhile, Kurt and Mercedes are hanging out more often. Their emotional similarities, and the way they do friendship similarly, become apparent in the way they deal with Unique. They are supportive of Unique because Unique is awesome. There is no question of sabotaging Unique or begrudging her talent.
- In contrast, Rachel (and Finn) are the only two in the audience at Nationals who do not join the standing ovation for Unique. (If you want to read more about Finn's (and Rachel's) douchebaggery here, paragraph 10 of calanthe_b's "Nationals" commentary is excellent.)
- Rachel doesn't clap for Unique winning MVP. Mercedes claps and smiles sweetly before turning to say something to Santana, and by then Mercedes is beaming. We don't see Kurt's reaction, but I have no doubt he is as ecstatic as he was when he first saw her on stage.
- One of the great things about Kurt is that he can feel genuine happiness for others, even when their happiness appears to come at his expense. (I say "appears," because, even in competition situations, I don't think Kurt sees happiness itself as a competition, or in limited supply, so someone else's happiness can't really come at his expense.)
- Mercedes, despite the whole Maria thing at the beginning of Season 3, is more like Kurt than Rachel is when it comes to #22. (Also, I don't think the whole Maria thing and the starting of The Troubletones was ever about competition for Mercedes. It was about having her voice heard, and about being put on a level playing field with Rachel. Because Mercedes was right - her Maria audition was way better than Rachel's. On a level playing field, in which Mr. Schue hadn't prejudiced the directors against Mercedes and in favor of Rachel, Mercedes would have gotten the role.)
- If Kurt ends up in New York, it seems inevitable that he and Rachel will drift apart unless Rachel undergoes a fundamental change. (Which is completely possible in college.) Up until now, Kurt has been giving all (most?) the support and doing all (most?) the emotional work in the friendship. You'd think he'd get tired of that eventually.
- He and Mercedes won't become best friends, but they'll stay in touch, and every time they see each other, it will be one of those easy things where it seems like no time has passed, because they are just on the same page emotionally about so many things.