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.
The fact is not like that and even that is difficult to make some gamblers not hunger to their money back, they gamble because there is something that the profit he sees is not just fun. This is what makes everyone an addict, they will always hope to win when betting. And I guess people who gamble are just for fun, so they are categorized as gambling addicts, even though they do gambling once a week.