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.
In theory all gamblers start this way and they think that they will not fall victim of greed.Then they start to gamble more and more to recoup the lost amount of money which ultimately leads to greed and 99% of the gamblers are destroyed emotionally speaking.The remaining 1% are those who are making a living from gambling.
Not just greed. Gambling is consider as addiction, and when you are into this to deep one thing follows another. Probably most of them are addicted to something else I think when they got gambling addiction they have some other to. Been in some gambling casinos and people are winning they get drink and everything else, when they lose they do the same. personally never seen professional gambler and really dont know how they live and what they do, maybe they just get up read papers and play bets.