Why work hard?

Why work hard? Why put in effort? I was asked this question by an 11-year old when I told him that it was good that he was working hard. Back then I couldn't find a good answer other than "you'll be happy you did it eventually". In hindsight, it's much more obvious. You work hard because you care. If you care about your grades, you put in effort to study. If you care about your health, you put in effort to exercise, sleep well, avoid bad food, etc.. If you care about someone, you put in effort to help them and make them happy. If you care about a product or service, you put in effort to make it right or improve it. If you care about something, it is self-evident that you would put in a lot of effort to do it right.