Many (good) players have irrational beliefs.
For example: "I'm the best player at this table, so I must win, and if I don't it's terribly unfair." and "If my hand is a favorite it should win, and it's terrible if it doesn't." And apparently many people believe that "players who play very badly and are morons and should not play so bad."

If you believe this, consciously or subconciously, you will be pissed every time one of the above happens.