he's mentally ill bro what do you expect
Maybe he's confusing Sunjeong with his Alpha? His brain craved love and care from his Alpha but since he's showing mental instability, he redirected that to Sunjeong who selflessly taking care of him.
it's not a maybe anymore, it is a fact he became possessive to Sunjeong because he crave love and care from his alpha since he was abandoned by his alpha. It was described that way in the chapter and because of how long Seok doesn't see or get touched by his alpha, he became mentally ill and since Sunjeong took care of Seok, he have come to develop possessiveness towards Sunjeong. It's not a spoiler btw this is just my observation
Wait, Seok is Omega right? But why he...