In a nutshell, because he didn't lose his personality to love.
In less nutshell, he's a man that's always been clear about his will and personality, never lied, never went soft, and can still be head over heels for MC. He's consistent. He can be a guide and a partner for Yunshin without being less in either role.
It's an uncommon situation and personality that I've never read, and I'm invested, even after that shit.
I didn't like him before, but i loved him when he fell for yunshin. It was adorable how he couldn't keep his hands to himself and was did things he wouldn't normally do(which annoyed his but he did them nonetheless). Also he's consistent with his kisses XD (i love when my fav couples kiss). All in all, he became more tolerable and likable once he fell in love and like the comment before me said, his personality didn't change.

Gen asking cause I don’t get it, I’m curious. Why do y’all like the ml?