Kurt tells himself that he's perfectly fine with Blaine and Tina being best friends.
He respects that the friendship is there. He would never tell Blaine not to be friends with her. Blaine and Tina's friendship remind him of how he and Rachel are. It's very nice that she's there for him and he's there for her.
It's unquestionably true that Tina still needs friends, especially as a lot of her friends graduated with Kurt.
The absence of Mike makes her loneliness ten times worse. Sure, there's the new New Directions, but he knows she and Artie have never gotten back quite to where they used to be and she and Sam barely talk at all.
Kurt and Tina used to be there for each other. They'd go shopping for cloth bolts and design outfits and sew on Kurt's mother's machine. They'd go through each other's closets for things to borrow. He remembers that she sang and danced backup for him - she always will, won't she? - and that she stood in (briefly) for his girlfriend when he was so frightened of what his father would think that he (briefly) pretended to be someone he wasn't.
She, and Mercedes, were always familiar ground, friendly faces, and warm blankets to cling to when comfort was needed. Spiritual chicken soup, as it were.
Kurt frowns at the thought of chicken soup. Tina made it for him once, too, when he was down with the flu for three days. She'd brought all his homework, the latest copy of Vogue, and a cup of that "special chicken soup". While it was a miracle worker of a cure, the mention of that soup – the cloudy pale broth, garlic, greens, delicate pearly chicken finely edged with ginger - now makes the taste-memory of it go sour.
He doesn't ask Blaine about how Tina's doing, either, not since Mr. Schuester's second wedding; especially after that very strange and infuriating conversation they had about why and how Blaine forgave Tina so easily. Kurt knows that if it had been him, ill and asleep under her bitter touch, he would have found that much, much harder to forgive. What she did to Blaine was assault, and it had been by the hands of a trusted friend. Kurt had experienced too many violations in his life to let that hard knowledge go. Kurt feels the anger for Blaine.
But there's something in Tina's face that stops him from confronting her again, after he did at the wedding that wasn't one. And though he stays silent and smiles at her politely for Blaine's sake, it still changes the weather between them. Kurt can't stop himself from keeping his distance. When he and Blaine walked down the hallway on the Monday morning after, arm in arm, he pulled away from her. He talked to her on the phone before leaving for Ohio, but he didn't tell her about his dad's medical appointment. Private, intimate details like that will not be information that she can have.
Kurt's deepest feeling is bewilderment. He wonders if he really knew her at all. He doesn't like that secrecy.
Tina used to be an open book. He, better than anyone else, understood why she dressed the way she did. He did it himself. Clothing is truth, and even when you're trying to conceal something, it will show in the way you dress. Now, he can't even trust the way she dresses, because he knows the way she's dressing isn't truthful.
They'd fought together in the trenches of loserdom. You should know everything about a person when you go through things like that. You should know. Now, something inside of her has wobbled loose and rattles around. It rolls around, unceasingly, like the metal ball in a pinball machine.
The most heartbreaking thing about it is the memories. Kurt remembers the first time he saw her in class, with her head bent over Frankenstein, the cobweb-fringed train of that black felt top hat falling about her jacket. Back then, they'd helped each other with wardrobes and troubles. Those days are gone.
He doesn't know if she'll go back to the way she used to be, or if this new Tina will always be here. There's a part of him that mourns, living side by side with his anger. Sadness lives there, too.