"Jung explains that he understands that feeling, because he also knows a person who's always bright and exciting."
At first I assumed it was Kyung Hwan, but then In Ho because:
Sul adds on that Jun is very fond of joking around and acting all cute to his parents, so in a way, his parents seem to prefer him. Jung tells her that if he was a parent, he would've liked Sul more. He thinks that being diligent is a better trait than being a jokester or cute one. He tells her, "In my eyes, Sul seems like the one who's a better daughter to her parents." Sul asks if he's serious, and he tells her he is, so there should be no reason for her to be so depressed. She smiles.
Jung might've been jealous of In Ho in the past, so he's referring to his situation and wished his father was more easygoing with him.