I think it's because Siwoo thought seriously about the two of them for so long and he realized that they are doomed because they're both alphas whose pheromones repel each other. So he did did it to make Byul wake up to reality and know it ain't all happy fun kissing and giggles! This shit srs!!
BUT, judging by Byuls reaction I'd say he is just like his dad and will manifest as an omega. Idk he didn't act as suffocated as an alpha who was exposed to dominating pheromones would.
Preach! Byul has "dominant omega" written all over him.
Then again, it's pretty much confirmed he's an alpha. With his height and pheromones.
But his character, his being bad at school stuff ... People liking him because he's adorable, rather than dashing.
And only Siwoo had the fine senses to notice the hidden omega traits early on. :3
He ran away from the kiss. WHY would you just dump your pheromones on him like that