A professional gambler is not allowed to be a witness before a Jewish religious court, because the Sages declared this practice to be a form of theft. The explanation that I’ve heard (I assume this is somewhere in the Talmud, but I don’t have time now to look for a citation) is that when you lay down money at a game of chance, you are not actually willing to lose it. Without that intention, the flow of money from the loser to the winner is morally tainted. The loser isn’t getting anything in exchange for the money that’s gone, and certainly didn’t intend it to be a gift.
(OK, some people who gamble are happy to give up a certain amount of cash in exchange for the thrill of the game, but those aren’t the people who keep casinos in business.)
Cf. this Globe article on college students and online poker (that page will
self-destruct vanish behind the paywall in 48 hours), which quotes one college junior as saying: “When I’m down just $100, I feel, oh, I just took a $100 bill out of my pocket and handed it to someone. Your tendency is to want to win it back.”