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what is it about poker that brings the worst out of many people, losing money?
It's called homeostasis.
Humans* try to maintain some sort of equilibrium.
With respect to human psychology homeostasis refers to an equilibrium between good and bad experiences. If you have many bad things happen to you you tend to want to pass on bad things to others (it's basically the reason why losers beat their wives. They have experiences of being inferior e.g. in the workplace so they need someone else to feel inferior in order to regain some semblance of 'balance'...or they try to force positive experiences which can take the form of smoking, gambling, doing drugs, etc.)
*you will actually find the concept of (chemical) homeostasis in all living things - from amoeba to higher lifeforms. All have mechanisms that work against any deviation from the norm. Emotions are - when you get right down to it - also a chemical reaction.