[They're both stalling, at this point. On some level, yes, dealing with Chuuya is tiresome and unpleasant, but on Dazai's end it's not so simple as that. He knows Chuuya better than most, knows the ins and outs of how he thinks and reacts to things. It wouldn't be so much work to poke and prod his former partner's buttons until he finds something sufficiently sensitive that merits an unlocking of the man's cage.

Why he's resisting is ultimately far more petty and childish than merely a lack of desire to complete their task. He hates pain, but perhaps what he hates even more is showing it to others, baring that patchwork hand-me-down quilt of his heart with all of its holes to she sunlight where all the missing spots become especially evident. He doesn't want to tell Chuuya something that will hurt. He especially doesn't want to tell Chuuya something that will only hurt him, that won't leave its own cactus burrs in Chuuya's skin to have to dig out carefully later.

The hollow jocularity of his voice drops as they reach the inevitable, though; his rambling tangents have taken them about as far as they can get, and now all that's left to do is decide where to place the incision into his own throat.]

...I know.

[He sighs. It has the sound of wind rustling through a canyon, stretching across the emptiness with a sound wave trying to be heard.]

I could tell you why I left, but I suspect it wouldn't be enough.

[It wouldn't really be a confession, when it's simply something he had no occasion to tell, and Chuuya would have had no desire to know. He turned against the Boss, left the Port Mafia, and lived to tell about it. Why would the reasons matter to someone like Chuuya, whose entire identity belongs to them?]

Not to mention, your resentment on the matter is about as far from a confession as my saying I want to die.