Gambling isnt forbidden in the Bible, and as many other things, it depends on ones attitude and practices as to when one has crossed a line into the danger zone.
First Id say that theres nothing wrong with occasional light gambling as a form of entertainment. It can be an enjoyment for people, much like other forms of entertainment. Spending a little bit of money to buy a lottery ticket or play the slots can be a fun adventure.
There are risks, however. They dont call it a one-armed bandit for nothing, and we all know the maxim, The house always wins. There are reasons.
Gambling can all too easily turn from mild entertainment to an overt form of greed (and greed IS forbidden by the Bible: Isa. 57.17; Mk. 7.21-23; Rom. 1.29, and a bunch more). Greed can come on us pretty easily and quickly bring us down. After all, the premise that makes gambling work so well is personal greed. So theres a problem.
God has no particular problems with wealth (for instance, he made Abraham and Daniel extremely wealthy people), but he cares a lot about how we got it and what our attitude towards it is. The Bible generally applauds working for what you have (Prov. 10.4; 13.11), and we must always beware that the accumulation of money doesnt control us (Eccl. 5.10). Money creates a desire for more money. People want it because they think it will bring happiness, some ease in life, and even a certain amount of peace and inner fulfillment. It will bring those things only if your heart is centered in Jesus, and your money is a tool you use to be a responsible steward for him.
Money has a strong potential for addiction, and with that comes abuse of other people, ignoring the needs of others, self-consumption, and the pursuit of luxury for selfish ends. Therein lies the danger and the lie of gambling. Just for fun can overnight turn into obsession, greed, abuse, and ignoring every principle we have ever learned about the responsible use of money and good stewardship. Hebrews 13.5 says, Keep your lives free from the love of money and be content with what you have, because God has said, Never will I leave you; never will I forsake you.
In addition, once I was asked, Whats the difference between gambling and investing in the stock market? Arent they both games of chance? Yes, and no. They are both games of chance, but the difference is this: Investing in a company is participating in the business a company does. In essence, we are loaning them money to grow their business. Of course there is a risk factor involved, since companies dont automatically grow. But investing is a business strategy of people helping each other. Gambling, on the other hand, is trying to get something for nothing. The casinos are trying to get your money by doing nothing for it: you put it in a machine or lay it on a table, and its theirs; you, by the same token, are putting your money out and hoping to walk away with a jackpot having done nothing to earn it. Thats the difference.