one thing that 122 has highlighted to me is that sskk's lack of understanding is the only thing that hindered them until now.
akutagawa sought to understand atsushi, but was still caught up with his own reason to live and could not comprehend a world where somebody else had a deadly ability but could be treated so kindly. once he understood atsushi, and understood the reasons he wished to fight or go on missions, he trusted him with rashomon and even sacrificed himself to ensure the 'better' man went ahead.
whereas atsushi, up until 122, meets akutagawa and focuses on why. why was akutagawa always like this. why is it always dazai that akutagawa focuses on. why does akutagawa even help atsushi. why did he save atsushi. it's never clicked in his mind that all of these things could be linked - or even, that akutagawa is just as human as atsushi is.
now the puzzle is complete in atsushi's mind, he understands. he knows the answers to why akutagawa has done all of the things that he did, and he knows why he and akutagawa are meant to work together. he gets it. he understands.