Emotional stress is one of the biggest thing that could actually affect anyone when gamble because you would need every part of your brain to be functional and active when playing and that's why even a simple glitch in the focus of mind can lead to so many errors while gambling and even mistakes so it's better to gamble when refresh and relaxed because you would be thinking straight and not doing crazy things just to get at the winning that might be getting so hard to come.
Gambling when a person is under stress can make it difficult for him to win as well as he will not be able to enjoy the real pleasure of gambling. Generally if we don't think of any source of income from gambling then there is no need for gambling during the time when we are busy with our work. Moreover, when a gambler is too busy or feeling tired, there is no better decision that can later have a major impact on his gambling life. Betting should never be done in a situation where a gambler is not completely mentally exhausted. Since gambling is now deeply associated with financial matters, every gambler should gamble in a completely stress-free environment or else he will never succeed in his goal in gambling.
There is a point, but I think gambling when you are stressed or tired does not mean that it will make it difficult for you to win because the name of luck can happen at any time without you expecting it but maybe a stressful or tired situation can make a person experience mistakes in terms of making decisions that can make the possibility of losing even greater, and yes it is true as you said that if we do not have the goal of earning from gambling then we better not force ourselves to gamble but with their decision to force the situation too much by gambling even though they are in a situation that does not allow it means they come with the intention and purpose to earn.
However, you are giving good advice by telling people not to gamble when they are tired or in a stressful situation due to other things in their life, but sometimes the advice will not apply to gamblers who are already addicted or it means that it is very difficult to give them advice even if it is the best for them.