So Matsutaro got scouted and reluctantly decided to do sumo in Tokyo. But he never wanted to do sumo in the first place, so his motivation-level is zero. He refuses to wake up for morning training. He even slugs his senpai in the end. But the professional sumo-wrestler Inokawa steps in to teach Matsutaro a lesson.
Why didn't I realize that sooner?
Wieso bin ich darauf 
nicht schon früher gekommen?
¿Por qué no lo pensé antes?
¿Por qué no lo pensé antes?
Por que não pensei nisso antes?