Not being contented with your win can be one of the reasons why you might end up losing more, you have a point on that. The main reason why a lot of people lose after winning is because they chase after more wins and as a result of this they end up throwing their stake and profits away. Gambling is a game of luck and whenever you get lucky it's important that you take advantage of it and stop chasing
Greed and un-satisfaction is what made them kept on gambling and lose all the money they won, it is not because they win. When we are discussing about why people chase their win we should not generalize that winning could ruin us. We ruin ourselves if we cant be contented. So, anyone gambler using this as an excuse is not ready to learn, he will keep making the same mistake if he dont realize he is the main reason for his losses and not his wins.