I think it’s because he’s basically a child LOL. He cannot read the room and I think yool implied it was ok for him to pursue him.
Although Yool isn't ready for a relationship he doesn't seem to mind Yechan following him around and telling him his feelings. I mean he did tell him straight up he is not ready for a relationship. Maybe he is hoping Yechan will give up eventually? So he has this attitude of well I was honest with you in saying I don't reciprocate your feelings (yet) so as long as I am honest and we are clear, keep shooting your shot kid. Also let's be honest, he is not doing s very good job at keeping boundaries between him and Yechan hahaha.
that's bc yechan is still immature and experiencing smth completely new to him so he doesn't really understand boundaries. sunyool already told him that he needs some time but yechan cannot really understand this bc yechans brain basically goes "i love you!!! please love me!!! look at me!! give me attention!!" yechan knows about sunyools situation but he does not understand it. he thinks that sunyool will be happy if yechan showers him with affection bc these feelings make him so happy so surely sunyool will be happy too. ofc that's not how it works but yechan doesnt get it bc love is smth completely new to him and he does not understand sunyools situation.
Real quick, why is Yechan being so open about his feelings again?? Since Yool recently got out of an emotionally exhausting relationship, it seems weird that Yechan is being very persistent with getting him to fall for him, and explicitly stating those intentions