Because if you are sad or depressed, for sure, there will be unnecessary consequences as you will be slave of your emotions.
Do remember, even if you are in calm position, when you place bet, your heart is already beating fast when the game is almost over.
How much more if you are in bad state and couldn't think clear? So you will put yourself in a terrible position where you may regret later on.
The point is that most of the time when people go to a physical casino they are happy with the intention of having fun, but when they are in the casino playing the face and behavior of the people who enter happily change a lot, this is because no one likes to lose, many Sometimes it's not just losing money that makes people sad, but rather having the feeling of being a loser, when they sit and think they lost, then they start to wonder why they lost while other people won and with that thought they start thinking that they are inferior compared to those people they were playing against.
When they play at the online casino and lose everything they start to feel guilty for not being able to beat the casino. The sad part about this is that many people, before starting to play, start to make a lot of plans about what they will do when they win, they enter the game with a lot of confidence but when they are playing and are losing, they become irritated and unhappy with the defeat. The game has a way of influencing people's feelings that there are few people who can really stay calm in the face of defeat.
There are many cases of people who were playing and talking and laughing with each other, but after playing for a long time and some of them losing, those who were losing started to insult and fight with the people who were winning. We can see this in football games, because there are so many games where you play against other people in person. That's why I always avoid any game with another person in person, because I don't want me to suffer any kind of physical aggression if I win, as I'm a peaceful person, so even if I lose, I'm not violent and I'm against violence.