There's something increadibly twisted about Dazai saying "certainly" with this much conviction, when he himself was struggling to find a reason to live in the Port Mafia.
Even worse when you remember that he'd declared his desire to watch the mafia burn in Stormbringer. Simply because he was bored of it all. Killing, death, schemes. All of it was the same. Everything became mundane.
Dazai never found any meaning in living in the Port Mafia, but here he is, promising Akutagawa the one thing he never got ahold of. Guaranteeing that Akutagawa would find it.
And the funny thing is, he did.
Dazai, in the end, did give Akutagawa a reason to live. But here's the catch: It's primarily tied to him. Not to the mafia, not to some high-end goal related to Akutagawa as a person, but to the suicidal executive.
And it makes sense, in hindsight.
Despite Dazai's visible efforts to make Akutagawa use his head more than his ability, he's the one (whether intentionally or unintentionally) who molded Akutagawa into this person, hungry for validation from his senior more than anything. And how you look at it (Dazai's intentions) determines if it was either a plan doomed from the start, or a perfectly calculated one.
Akutagawa would never want to seek more than what he was taught to seek. Would never stop being Dazai's lapdog. Would never grow a conscience that defines him, shape him into his own person. Would never hope for more than the Port Mafia, being Dazai's shadow.
In turn, Dazai would never need to worry about Akutagawa's intentions, only his executions. Something far easier to discipline someone over.
From the very beginning, Akutagawa was a wild card Dazai needed to moderate. So he made his approval all he should seek. He made him strive for nothing more.
He made Dazai all Akutagawa is.
And once he fulfills his wish to end his life, Akutagawa will be gone as well.
