Often traders have bad timing, and not enough capital to survive the shake out. Too many traders perceive futures markets as an intuitive arena. The inability to distinguish between price fluctuations which reflect a fundamental change and those which represent an interim change often causes losses. Not following a disciplined trading program leads to accepting large losses and small profits. Many traders do not define offensive and defensive plans when an initial position is taken.
Emotion makes many traders hold a loser too long.
Many traders don't discipline themselves to take small losses and big gains. Too many traders are under-financed, and get washed out at the extremes. Greed causes some traders to allow profits to dwindle into losses while hoping for larger profits. This is really a lack of discipline. Also, having too many trades on at one time and overtrading for the amount of capital involved can stem from greed.