Sebastian was visiting Ohio (visiting his grandparent or something) and was in the mall during the Barbra-vention for Rachel in Born This Way. He sees Kurt and is immediately attracted to him and his sass/bitchiness. Then, a few months later, Sebastian moves to Ohio and attends Dalton, meets Blaine, the whole thing from The First Time. He officially meets Kurt as Blaine's boyfriend, but Sebastian is very confused that this Kurt that he's met looks like the Kurt he saw, but doesn't act like him. But he finds that he sees glimpses of the old Kurt when they bicker and realizes that Kurt's lack of attitude is because of Blaine.
Somehow they fall in love over time (that's fuzzy to me) and they move in together in New York, I think. And Sebastian shares the journal entries he'd written at the time with Kurt, which basically tell the above story. (Note: This is not Because of You by LadyDivine91. Sebastian is actively letting Kurt read the journal. Kurt didn't just find it.)
Any help would be greatly appreciated.