It’s commentary on the cycle of abuse and how other peoples actions effect others.
That’s why.
Duh.
ok wait but does no one question why everything repeats itself in bsd??
like why does (which I now found out in fanon is called zenki skk)
fuku and mori turn into
dazai and chuuya which turns into
atsushi x akutagawa??
like is the cycle just suppose to keep going? who's going to stop it? like when are they going to get the help that they need to change as people and stop this cycle of abuse? are they going to die with their morals/lives being like this? with no change? is that just the way things are? are they just suppose to keep living under our predecessors failings? or are they going to come together to stop this cycle of abuse and help each other? are other characters going to get involved in the story later on? are they going to just continue right where atsushi and aku left off? am I thinking way too much into this? but why did it happen over three generations? and why-