At the age of 4 a certain doctor told him to give up his dreams of being a hero.
He told him, that it's likely that his quirk won't ever manifest. But Izuku knew that he had a quirk, but no one wanted to believe him. A new, never before seen quirk, had appeared in the boy.
"Hah, I bet that he only pretends he has a quirk"
"Show us your quirk, oh wait. You can't haha."
"He is just being delusional. He is fooling us because he doesn't want us to know that he is a quirkless loser!"
"Don't even think about going to U.A. DEKU! Or else I will kill you!
He will prove them wrong.