I read a lot of comments saying that Seul is trash and all that BUTTTT let us not forget that he almost lost Junie. Junie rejected him a lot of tines and Seulie probably knows that he messed up big time before. I believe the reason why he is like this, obsessive, is because he is afraid Junie will realize that he is not perfect and will leave him. As we all know, Jun's idea of Seul is way too perfect. Imagine your partner having that kind of idea about you... You'll panic and overthink for sure.
Also, a lot of people are getting mad at Seul for rejecting Jun. DID YOU ALL FORGET THAT HE WAS STRAIGHT? Of course he would be weirded out by it at first. He had to understand his feelings first and he need to fully accept the idea of dating another guy. It's not like you will get a confession from someone with the same sex and just accept it because you enjoyed hanging out with them in real life. You'll probably feel weird too... I think people romanticize BL too much that they forget that love between same sex is not full of sunshines and rainbows in real world. It's confusing and can be scary in real life. It's not just fluff.