It's a human nature to be greedy sometimes, but it's a must that you can still think of what should be the right thing do, is it to go beyond your limit or stick to your set plan? If you chose the latter then you're a kind of gambler who never let greed influence you just because you are winning/losing.
Yeah, humans are greedy by nature and that's the main cause of our losses. Sometimes when we are under the influence of greed then our senses somehow stop working properly and that's why be tend to make decisions in hurry in order to earn a lot of money in short time.
There are few exceptional guys or you can say gamblers who know their limits and stick to their plans and strategies. They are quite a few in number but they are the ones who don't really face huge losses even on days when their luck doesn't favor them a lot. It's way better to not win then to lose, and those gamblers choose not to lose first and that's why they are happy in their lives.