Izuku never really knew why Kacchan allowed his quirk to control him, to let the praise get to his head. he's always been that one brave kid in town who'd willingly go through the dangerous part of town just to get to someone else's home.
He never questioned the days that Kacchan would come to school tired, and resting his head against the table. The teachers never waking him up throughout elementary and middle school, he was always ahead by a month anyways.
He didn't question it till Kacchan was hit by a permanent de-aging quirk that left him at the tender age of Three. Curled up in a ball, face covered by his hands.
Counting till he stopped at Thirty.