You've gained the approval of Dazai without seeming to try, Akutagawa seems to feel quite envious of your achievement. Indeed, he has been striving for quite some time to seek said approval and has failed to gain it, causing him to feel resentful towards you.

"i am unable to ascertain as to why dazai thinks good of you; you're weak." Akutagawa muttered, casting his gaze away from you as if your very presence disgusted him.