Two versions of fear impair traders: the fear of losing money and the fear of missing opportunity.

Out of fear of losing money, traders will avoid buying weak markets or selling strong ones; they will stop out of long trades on weakness and exit short trades on strength.

Out of fear of missing opportunity, traders will buy markets when they're up and sell them when they're down.

Both forms of fear have negative expected return, particularly in low volatility market conditions, when moves are least likely to extend. Of course, it's these same low volatility conditions that lead traders to lament that there are no market moves and no way to make money.