Is he naiv, on drugs or just happy. What a good question. Is it possible to be so happy with a so tragic past. Nobody knows his past except of him. He seems to feel no pain. He is just a random quirkless freak that has a bright smile. Found was he on the dirtiest beach with ten year old. No memory, no quirk, no name that meant no chance of finding out his past.
Not like Izuku would care. He left his past behind and try's to do anything to run away from it. Can he escape the grip of the hero commission that left scares in his past and childhood