The theory is;
If you want to gamble, see it more like you are paying the casino or gambling platform to have some fun (just like you do to go watch some movies in cinema), which if you are paying for anything, you are not meant to expect it back. The good thing however is that, you can actually end up being paid more for having that fun if you get lucky, but the chances of seeing that happen is pretty slim. In that light, if you cannot actually live with doing what I said above, then you should not be gambling at all.
This theory of yours doesn't seem realistic to me because gambling most time is for earning and not for fun as you propose. Allow me to say this; gambling for money is the only means you can earn from gambling else gambling for fun as the primary aim will never generate money from that fun. So, gamble as if you want money and the end result will surely be profitable.
You cannot compare gambling and casino with movie watching etc. You pay for the movie and then enjoy it for some 2 hours. You do not enjoy by placing your money in gambling game and losing in a matter of seconds. Gambling is played with the aim of getting quick money and I don't think people only play it for Fun only.