His possessiveness might stem from their childhood trauma. They both latched on to one another for support when they were both drowning in pain and trauma. He wants to control everything from what Jeaun eats to what time he wakes up. He confessed to manipulating things so that Jeaun feared Alphas, the one thing he could never be. I honestly feel so bad for all three of them.
However jihan & jeaun’s relationship will never work and will be full of doubts, jealousy, unhealthy codependency, and fear of it ending any minute w/Siwan in the picture
Seeing Jeaun admitting that it pained him to think about having to tell Siwan to back off just solidified that their whole fated pair is strong. Unless Jihan overcomes his obsessiveness, codependency, insecurities and somehow invents a medicine that breaks fated pairs I don’t see their relationship working.

We saw the beginning scene of chapter one. Jihan is possessive and obsessive. He’s creepy, and it looks as though he tries to force himself on Jaeun. He’s jealous yeah, but the fact that he’s trying to control everything in Jaeun’s life isn’t healthy. He’s even admitted to trying to make Jaeun afraid of alphas.