okay so personally, i value friendships a lot more than relationships so if i have a really good friend, we are practically bros...but in kdramas most of the time the best friend wins my heart, cuz of the way they know about the little things, there is so much trust, and they might have alr had conflicts during friendship so they know how to deal with it if it happens when they are dating.
gunho is a really good guy and if sunjae and haeyi weren't bsf i would be extremely conflicted
also, unlike many other cases of bsf being in love acting kinda toxic, sunjae doesn't even show a single such sign, instead he is just so supportive of haeyi in every way possible, he trusts her in every way possible, doesn't put a lil bit of pressure on her to tell the truth, until she is comfortable... even growing up in a toxic family he is so lovable to haeyi
now coming to gunho, haeyi is a really ambitious and mature girl, while gunho is more on the playful side, and maybe people say opposite attracts, but sooner or later haeyi's ambitions and family issues would create a conflict between the two
also, sunjae's only source of happiness is haeyi, and the day she leaves him, will be the last day of this drama for me
also, one thing i don't like about gunho is thathe always tries to come between the two, i get that u like her, but sunjae is her bsf ... u r doing her bad if u want her to leave her bsf who is always caring to her