Just imagine if everyone going to the casino and have the positive mindset that they're will go home with the jackpot, casino's will be bankrupt by now. Lol. It doesn't work that way in gambling because everything is based on our luck. Heck, I even some negative thoughts when going to casino, specially if my friends wanted to go and I'm reluctant. But it turns out to be my lucky day!!!
If there are a lot of people going to the casinos, the probability of winning a jackpot increases but also the fund of money to the casino increases too, so it won't be bankrupt, in my opinion. You can never control luck and sometimes I experienced on what you posted, having that negativity and turning out positive. It's like making karma to a positive, or something like that.
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No one is actually saying there is harm in thinking positively whether you want to gamble or do anything in life, the choice is actually yours. However, where there is a problem is when you are not actually being realistic and you are focusing more on the positivity than trying to be truthful to yourself that this is just gambling, a game of luck and you need to set a limit for yourself.
A lot of people have gotten carried away by thinking positively that they always end up losing their steps and getting so deep into gambling that they will end up finding it pretty hard to get out.
How could there be any harm? I'm just curious because there are things in your mind that could make you feel happier just by thinking positively. It's a matter of how much you want to stay positive and never looking at the negative side of life. As long as you are following the rules of yourself (if you have set managed funds) you should be fine.