Actually Jooin was going to say something after the talk with his mother but it was Cain who stop him from giving an answer, but I think it was for the best because Jooin isn't ready at all for a relationship at the moment. Plus Cain said he's glad Jooin came back to talk to him instead of leaving things undone.
I guess that's a way to see it. Letting him go with this last part was the best way to end it in my opinion. They're related to a past life and cain's way too aware of it to just let go. We also have the information of Cain who never had been able to give a meaning to his life and just living to exist in a world where he's the 3rd spectator ever since he was young so ? Kinda was good in my opinion that he'd go remembering of jooin

Not Jooin rejecting Cain then calling his brother to find him again just to reject him more intensely, Thank God leaks are here and the author gains nothing from this rotten milk "please stop reading me" ahh bl cause what