Dazai always thought Chuuya was useless and he thought he was better of without him so he told him that he wanted to break up with him

Dazai: leave. I dont want to see your useless face

Chuuya had no choice but to leave and he left and after just a week Dazai was feeling depressed, empty, lonely and miserable without Chuuya

after a month he saw Chuuya again