
I thinks its because that seunghyun was suddenly caught up in his unrequited love(bcuz yk the fountain pain) and just drag section chief to it and their current relationship(cuz he was the ex) And then comes section chief being frank about his words like it is simple which hurt seunghyun (it was about his past love).... Maybe?? Thats how i see it.. I am also confuse lol

I guess they fought because Seunghyun avoided him for some time and when he asked why Seunghyun answered that he did that because he realized that the Chief was his first love's ex, so chief felt like Seunghyun's blaming him or that Seunghyun can't get over his first love, which isn't really true, he was just feeling conflicted and wanted to see how they're gonna deal with that fact but because Chief can't really understand Seunghyun's troubles, he dismissed it and invalidated his past pain which obviously hurt Seunghyun and made him snap back at the chief. Now they're fighting.
It's definitely cause I'm stupid but what exactly are they fighting about?