Would you say it's simple? Saying, "Hey, just quit" to someone? No.
No, because the human brain is wired to think in that manner.
Let say there is a gambler who have received payment today and first he kept aside fund for his daily expenses, and left over with a reasonable amount which is right if can be saved for his child education, insurance policy, emergency, etc.
instead of saving he goes to casino for some day until he busts his leftover payment.
first day he won a small amount and gone home feeling happy and he didn't purchased anything for his wife or children by that winning amount.
next day he wins.
next day he loses his 10% amount in one game, now he is becoming a aggressive gambler, he started betting 5%-20% of his bankroll in one bet.
after playing some bet he loses his all money which was left that day when he received his payment.
after losing he becomes so angry at him and start to think ("i always lose all of my money on these stupid games, i will never gamble again") after some times his mind becomes calm and he start to think (oh! i was lost because of that mistake... I will try playing and recovering my all lost money on next payment day)
this cycle repeat in his mind and he becomes an addict who loses his hard earned money playing games, risking his families future. keeping them restricted to even a small luxury like giving a fake but beautiful ring to his wife, gifting his children with a little expensive pen which they can show to their friends in school and make them feel jealous, etc.