I think Dazai, while young, became attached to Natsume when Natsume was lonely and so was he (which is why he calls him Sensei in the Dark Era, like the protagonist in Kokoro); that Natsume brought Dazai to Mori after Dazai attempted suicide (which is why Dazai never touches him in Bar Lupin, why Natsume follows him from the Port Mafia to his meeting with Taneda and then to the Agency, why Dazai knows Natsume well enough to allude to his history to Tanizaki and to recieve the jump drive Natsume brings him in the Cannibalism arc— Dazai knows him); and that Natsume performed a partial skill transfer similar to Kyouka's mother to ensure it could not come that close ever again (and that's why Dazai is one of the few if not only touch/contact-based skill users whose is involuntary).

I think No Longer Human isn't merely nullification, I think (similar to how Bram's skill reverses death but through the framework of vampirism and Atsushi's slices skills and regenerates in addition to his transformations), there is more to Dazai's skill such that it precludes him from dying (which is why he can stand in front of a hail of bullets and be thrown by Chuuya through several buildings in the Port Mafia, why he can't seem to commit suicide successfully, why Kunikida and Chuuya remark on his preternatural inability to die, why Dazai says to both Atsushi and Fyodor with such certainty "you can't kill me," and, cheekily, why Kunikida tells Dazai in 55 Minutes "You're no longer human! You've lost that right!" when Dazai says he finally managed to die, however briefly).

I think Natsume's skill bends reality in the shape of the narrative structure over which he had such mastery and he decided that whoever he failed to snatch from the cyclical jaws of misery and suicide before (again, borrowing from Kokoro), he would not let this boy die; and how cruel that must have felt at first (which is why Dazai hates pain— he's been denied the easiest escape from it that he knows), but how reality bent to weave a narrative such that Dazai could be saved again and again in such a way that might shatter Koroko's inescapable loneliness (this is why, in 55 Minutes, it's implied HG Wells might not have existed, might have been an apparition from an earlier era, as if reality itself rippled space and time to reconjure someone who made sense to protect Dazai when it seemed unlikely anyone else could; why Akutagawa happened to have business there; why Akutagawa and Atsushi happened to cross paths).

It's well known that the titular cat in I Am a Cat is a calico. But, the story was based on Natsume's IRL cat, who was a black, striped cat. And aren't Dazai's bandages a little like stripes?