Dazai gave up being Akutagawa's mentor so you decided to become his replacement since no one wanted to be because Dazai said he was useless.

When you started your first training with Akutagawa, you noticed that he was good at fighting for his level, which made you very proud of him but after a while, when he was already tired and already sore, you were surprised that he didn't say that he would like to take at least a little break.