Well, they have to cut on the novel a bit to make this manhua and at this point in time his relation with Joonghyuk isn't really explained but if you want an explanation borderline spoiler here it is :
You see, our suicidal protagonist sees Joonghyuk as his brother, someone very close who helped him during hard times since it felt like Joonghyuk was just next to him when reading the novel. However it doesn't mean MC loves Joonghyuk unconditionally : I don't know if you have siblings but since Joonghyuk is a bit of a brother to him he has a rocky relationship with him. So even if does care about Joonghyuk, there is also a lot of things he actually dislike in Joonghyuk, especially his thought process. So of course he wouldn't like being "assimilated" with him through the third stage of his skill. Tbh I would hate to be in this situation with my sister ^^
Well I hope I resolved your question and this is just how I understood from both the reading the novel and the manhua so I could be in the wrong.
Thank you! That actually really did help!! And yeah I wouldn't want to be my sister either (￣∇￣")
At the end of chapter 53, why does Dokja say he "hates this" when he's in Joonghyuk's point of view?