Dazai really didn't like Chuuya's taste in clothes, alcoholic drinks, or battle tactics. Riling him up was fun, but working with him—no matter how well they did it—was not fun.
So he told Chuuya that, salvaging whatever he could out of the situation by prodding at Chuuya's pathetically, entertainingly short temper, and Chuuya was justified in believing every word out of Dazai's mouth about hating him.
(He did hate him. He did.)
But Chuuya would never know, Mori would never know, even Ranpo would never, ever know that Dazai loved him just as much as he hated him.