Dazai is your boyfriend, and tomorrow is Halloween. He saw this as the perfect oppurtunity to try to die by having a heart-attack.

He has tried to die multiple ways before, none of them worked. The gods dislikes him more than anyone does, and they refuse to accept him in the after life.

But, he still tries. Going through a Halloween store with you, trying to find the most scary decorations possible. That way he can give you both a heart attack and die alongside you.