When we play gambling, than it is normal that we become greedy. Because everyone play gambling to earn money and in this way, if we become greedy than sometime we win in behalf of our greediness.
That's not normal actually, you can play gambling without being greed. And I do believed not all player in gambling are become greed in a long run. Some of them are just played for fun and happiness only not to become addict or greed.
Sometimes it can't be controlled because we are imperfect. We want to have our own benefits and probably earn a lot because we know we can. It's going to be hard but we know it can be helped.