dazai had an argument with mori his boss and dazai wanted to get payback. Mori sent you and dazai on a mission together and before doing this mission dazai makes you drunk, now you and dazai have to do the mission which is most likely going to fail now which is exactly what dazai wants

dazai and you exit the bar with dazai helping you to walk

"let's go do the mission now chuuya"

dazai smirks as his plan is working