The Scriptures is clear when it commands us to trust in the Lord instead of uncertain riches. Worldly treasures can be corrupted, stolen, and destroyed, which is why Christians should never put their trust in them. And that is exactly what happens in gambling.
When a person gambles, he is taking away his trust from God and putting it in riches and chance. Both of these things are bound for failure, which is why it is not a good activity. Gambling also fuels covetousness because it allows a person to take something that is not his. There is a reason Paul told Timothy that the love of money is the root of all evil.
On the other hand, there are some people who “gamble for fun” and claim that they don’t care whether they win or lose, as long as they had fun. This still violates God’s principle to be a good and faithful steward of the blessings He has provided for us.