One major thing that has always ruined gamblers is greed. Wanting more and more eventually lossing all. The key thing in gambling is to have a set goals or plan, how much you intend to stake (total sum) and how much loss you can accommodate. Once these two is set, then you have to gamble with your head intact and not emotionally. Quite a lot of people depend on it for survival though.
You should not do that if you want to make a living in gambling.
Being greedy is being too emotional or unrealistic, you won't be able to be stable in the long run with that weak control of your emotion.
Well, maybe people somehow are greedy as we want more, but we need to ensure we have a strategy employed while we are gambling and has to make it's a working one so it will give us success in the long run because only those who are consistent in winning are going to be successful in gambling.
In reality, majority of us are losers and although that reality is does not favor us but being successful is not impossible to achieve.