Sunghoon basically have to explain what society is, and as man of society, there are rules to how one must conduct among your peers. Exchanging freedom to gain other kind of advantage, like security, help between people and so on. I wonder as Sunghoon continue to interact and explain things to Sungui, he'll reflect how phantom & human will live in peace, as he'll be in an important place being Rangii's (future) husband. Most phantom on Rangii's "side" want to be free IIRC, that's why they follow Rangii. Whether Sunghoon like or not, he'll have to become some sort of diplomat between Rangii's subjects and Korean's people if he wants to stay at Rangii's side (and avoid future bloodshed).