Being human . . . it would mean not being scared anymore. No more being hungry, or worrying that you'll hurt someone, or lose yourself.
But it's always seemed that Zero's happiness . . . that it's not a selfish thing. That it means . . . that the people you care for are safe. That they're happy, too.
All you wanted from me was to see me smile. If there's no one at home, that can smile for you . . . even if you're human, would you be happy?