라 돌체 비타

Uh, easily.

Sebastian had no societal obligation to be even remotely nice to Kurt, they were rivals in show choir and for Blaine, not to mention that for every insult Sebastian threw at Kurt, Kurt threw one back. He never seemed hurt by the insult just annoyed which added to Sebastian's lack of realization that his words could hurt people (until Karofsky).

Blaine was Kurt's boyfriend and the expectations that come with that are to be nice, caring, and loving. Blaine tries to force Kurt to have their first time in the back of his car in the parking lot of a bar, despite knowing that Kurt wanted it to be special and romantic. He then yells at Kurt for not being 'Spontaneous and Fun', which yeah he was drunk but that doesn't give him an excuse. He later cheats on, fully cheats on not just the texting thing, on Kurt and tries to blame it on Kurt not being there for him.

Sebastian may have been mean to Kurt but he apologizes and plans to be better, unlike main cast characters we don't see much of that since he's not on screen that often but he is present for the proposal and seems at least civil with the New Directions.

See the idea behind the argument against Kurtbastian is that because Sebastian was mean, something he acknowledged, apologizes for, and promises to improve upon, means that he cannot in any way be shipped with Kurt.

Yet the man that tried to force himself onto him in the back of a car, cheated on him, blamed him for said cheating, tried to guilt him when he made improvements on himself, tried to control who he could be friends with, is considered his soulmate???

'you and me' by Diana Boier