It is necessary to keep self control in poker games
Self control is one of the twin towers to win the poker game (the other is the correct poker strategy). Knowing the right move for the situation is absolutely irrelevant if you do not have the self control to run it. If, for example, you have a well established and successful list of starting hands to suit your situation, but you\'re too impatient to fold hands that are not on your list, so hardly matter if you have a list of the right starting hands in the first place.
Losing that puts control of self at risk. In its most obvious manifestation of a player loses a lot and then goes on tilt - unable to keep his inner demons from corrupting his optimal poker game. The deeper he sinks into the red, the less self control he has. He enters what called a "death spiral" of bad decisions, more losses, and the control decreases - which ends only after running through his entire bankroll or leave the room.