i could think of two main things that make sense to me depending on the scenario: either insecure about the relationship to the point where it becomes a problem to his partner, or his quirks becoming too much.

on the first one it relates to how i think seonghwa at times keeps too much inside and this could prevent him from bringing up uncomfortable things and the second one is like... no one who leaves dirty clothes on the floor instead of the hamper would be suitable for a long term relationship with him lmao