Because trading is an uncertain game of probabilities filled with uncertain vagaries, an overly inflated ego or a fragile ego can easily get smashed. Defending the ego uses up unnecessary energy, distorts perception, and sooner or later, will destroy the trading. If your self esteem rises and falls with your trading results, you and your trading are in trouble. Self concept has to be strong and durable and not at the mercy of the current, last, or next trade. We need to check our egos at the door when we start to trade.
Uncertainty is central to trading.
If we add the uncertainty of our own self image into the mix of the unknowable endemic to trading, we're in for certain trouble sooner or later. The ego doesn't want to be proven wrong. Hesitating before putting on a trade. The ego wants reassurance before it begins. The ego wants to prove itself big time.