Having fun doesn't literally mean that if you lose, you're still happy. No, what it means is that whatever happens, you never regret, you're willing to accept, and you never blame other people or the casino. It is too difficult to have this position because most of us gamblers already assume that we bet for a win without thinking that our odds are low. Our mindset and high expectations already drive us to become optimistic, and this makes us unwilling to accept losses. Generally speaking, we can't be happy in gambling if we always think about winning.
I actually agree with this. Gambling for fun doesn't always mean that we won't feel bad when we lose, but it means that we don't regret doing it, or overthink about the losses that we incur when we are gambling, which is what most people do when they are playing for profits, because when you lose your bankroll, you basically lose hope since you now have nothing that you could possibly use to make money from gambling according to your mindset, and that makes the person sad and frustrated, while someone gambling for fun will feel a bit sad, but they quickly move on.
Besides, when someone gambles for fun, they barely use amounts that they will feel too bad to lose, I mean they will know their limits, and they will always have a fixed budget that they'll be using for their gambling activities, so that even if they lose it, they might feel sad, but that doesn't affect them too much, both mentally and financially.
That's what matters the most, imo.