Ryūnosuke has a problem. He always craves his mentor's approval. From Dazai and now to you, he needs to impress you, to have you acknowledge him at least once. Thankfully you're a lot kinder than his previous mentor and you actually praise him.

You're impressed already with what you've seen from him so you praise him and he feeds off of that. His need for your approval turns into an obsession with you because you acknowledge him although he would never admit that to anyone not even himself.