Sometimes, you become associated with one you are not meant to be associated with. It happens. A great mistake. That is all.
Of course, it makes more sense that Nakahara, Chuuya would be still close to Dazai, Osamu. After all, they used to be the most threatening duo when Dazai was still in the Port Mafia as an executive.
Obviously, their relationship had changed after the betrayal, but, still, Nakahara couldn't stay away it seemed. A dangerous game the executive played, dating the enemy. Of course, they had to keep this a secret, as neither organization would quite so approve of them still being together, even if they worked together well in battle still. This could ever so easily be a threat to both sides.
Keeping secrets isn't the absolute easiest, especially when the secret is an entire relationship. Sometimes keeping a relationship a secret can be even more negative of an impact on the people within it. For example, do you really think Nakahara feels loved when he cannot even hold his lover's hand outside of their bedroom?
Dazai and Chuuya have FINALLY confessed their feelings for one another! It's been a couple of months since they started dating, each going on with their lives, but, with a new blooming relationship between the two. But unfortunately, when your people like Dazai and Chuuya, not everything stays cheerful. Chuuya develops anxiety over the thought of Dazai leaving him again but decides not to let his mackerel of a boyfriend know for fear of hurting Dazai's feelings for mistrusting him. He does not want some 'anxiety' to break the relationship he and Dazai have been longing to begin. Of course, Chuuya should have known that he couldn't hide his inner thoughts from his intelligent (man's literally a detective!) boyfriend. Will Dazai be able to figure out what's wrong with his adorable chibi? Or will trust begin to break between the two?