I don't know why everyone keeps saying "he has to win." In my opinion the reason the books or bookies win is because people simply can't stop betting.
If anyone is up $100 on Saturday morning, they bet at least that much thinking they are up. They win that bet they press again with all their winnings on the afternoon games. Win that bet, press again, lose that afternoon bet, they are now "down" even though they are even and chase the lost won money (did I lose anyone yet?).
People, most people simply can't stop betting until they are down and that my friends is why the bookies win.