one of the (many) reasons why this exchange makes me so emotional is that Dazai knows very well the feeling of only ever being used and taken advantage of as a tool, and not knowing anything other than that life. He understands that Sigma can’t yet comprehend the notion of someone wanting you without any ulterior motives, of people just having natural relationships that don’t involve wanting to get something out of each other, because that was his life before he met Oda, who was a “balm for the soul” (in his own words) for him; hence, he doesn’t try to explain it to Sigma at first. But he does relent once Sigma takes Dazai’s initial silence as him being unworthy for an explanation... He doesn’t go into more detail as to why “understanding would be difficult”, because that would require opening up about himself which is something Dazai is still far from ready for, and because he knows Sigma wouldn’t be able to fully understand or accept all of this yet, but just the two reasons he does give for choosing him are already causing Sigma to start to rethink everything he’s ever known about the world and how people treat each other.

Dazai chose him to use his ability, just like everyone who’s always used Sigma has -- but he also chose him to save his life, and to help the agency, people he cares about and wants to help for completely selfless reasons, without expecting anything in return (sure, Dazai jokes about him being on the agency’s payroll, but that’s just his usual deflecting to avoid admitting that he cares :’). This is completely foreign behavior in Sigma’s world, where everyone has to use each other in order to survive, but he’s realizing that Dazai is different from them: he’s realizing what it feels like to be wanted simply out of compassion and love, and not because of how valuable he may be. Sigma isn’t just any other ordinary orphan Dazai saves; he wants to save him particularly because he empathizes with him and sees his old, hurt, jaded self in him, and he’s slowly becoming the balm for the soul for Sigma that Oda was for himself back then, which is truly touching.