They fell for each other. She has the elegance of a doe yet she fell for this stray dog who came from underground.
She made a mistake and now she pays the price after he began to live in a world of ignorance to alleviate his aching heart.
Alternatively: She fell in love with Dazai Osamu and thought she'd have the strenght to die with him.
She loses herself in her own little word, waiting for him to return home.
He loses himself, watching her change for him.
Alternatively; the girl makes a big statement she can't easily take back and Dazai chooses to play ignorant for one night.