It's called passion my friend but I guess you had that in mind as well? Passion makes even a tough job enjoyable. In my 14 years at several banks and hedge funds as trader have I not looked even once on my watch to check when to go home. Not even one single time. And I don't remember when I was ever sick (I believe sometimes, not always, sickness highly correlates with our emotional and psychological state of being)
Why does any business person with a successful business keep working? No difference.

. But that's what I'd postulate. I'm not that person so I can't say more.