why?

the author wanted to make a separate story about dojin and hye-sung so they made love is an illusion

I read love is an illusion first too and was looking for both dojin and hyesung when I read this since the author mentioned that the they where originally from this story. I was really glad when I spotted Dojin, and feel like a complete idiot when I realized that hyesung was that guy.
Anyways, the primary couple for this series also appears in a cameo in love is an illusion during the hospital scene

http://www.mangago.me/read-manga/love_is_an_illusion/uu/love_is_an_illusion-chapter-20/5/

I was wondering the same thing lol. I was so confused like uh did she just put them in there just because. But so what I understand then this story came before Love is an Illusion right ?

Wow haha this is what they called real detective I also didn't recognize hyesung for so long hahaha felt so stupid

Right! It was probably because his name was was spelled differently on the first chapter (hyeseong vs hyesung), I even recognized kyungsoo's name before I recognized his and his body build was also different from what we're used to.

Omg this is soo cute I love both of these couples TnT

OMG, Minjae’s pregnant, and Hyeongjo carrying their baby.