Why continue to Gamble?

I know why people gamble. You play the slots, try to be the one to hit the jackpot. You move to Hollywood, try to be one of the few who gets on the big screen. You date, try to find the person you want to spend your life with. It's easy to understand the human motivation to take a chance. The real question is.....Why, once a person wins, does she continue to gamble?

A man goes to Las Vegas. He bets everything he has on one roll. He wins and has everything he has ever wanted. He has spent his life trying to make a roll just like this one. He knows he was lucky. He knows the house usually wins. So why does he continue to gamble after he's already won?

A young graduate scores the job of her dreams. A few years past and she believes that she can do better, be hired to do something more. Why does the girl give up something she has worked her whole life to achieve? Why does she give up the perfect job on the slim assumption that there's something better?

Two people fall in love. They have been looking for each other high and low. They both know that the other is a perfect match, that they want to spend forever with each other, that they were lucky to find each other in the first place. A few months pass and the man decides to break up with her. He is afraid he is missing out on something more, something greater. Why would a person assume there's someone better after they have already found the best?

Why gamble when you've already won?