Grandpa is so obsessed over his grandson being gay. I don't know what his deal is and why he is so obsessed with this. I swear if he has some kind of backstory where he had a gay lover that betrayed him or something I'm gonna lose it.
I have a theory that our rapper Seungmin is actually a detective and has been trying to get to the root of the happy drug for a while now. I think he knew about Jaeyoung suffering from the drug as well and learned it was probably connected to the original group member Jun. So Seungmin decided he would come on the scene out of nowhere and quickly became a star in rapping. He did this to meet Jaeyoung and getting his attention in hopes of getting closer to learning more about Jun, who is in the upper ranks of distributing the happy drug. This would explain why he chose the exact same name "Happy" and instantly wanted to work him after only meeting him one time.
I don't think he expected to fall so hard for Jaeyoung, but as we all have seen, Seungmin has definitely fallen hard for him and so things just kind of happened in an unexpected way.