Sometimes it is not greed though but also a fear to lose more.
It is most likely the fear of losing more. Some people may see it as greedy because the player keeps tossing in more. But for some who have been and experienced the same thing, they know the player is trying to gain back what he/she lost.
When they try to gain back what they lost before, that will not be easy for them because that can be the other loss of money for them. Even if you can recover what you lose, that will not fully recover because you will still have a loss in the end. If you can gain back you lose, you will be tempted and think that you will win more in the next amount to lead you to get another loss. Whenever you won, that would be your time to quit gambling and not continue gambling because that will be the way for a gambler to have another loss.