Overtrading is a major obstacle for profitability. People tend to overtrade when they don’t have a plan for the trading day/week.
If overtrading is a major issue for professional traders, lack of discipline is a major issue for developing traders.
Many people just don’t have the necessary knowledge to trade profitable. Even more have the knowledge, but don’t apply it on a regular basis and don’t prepare for the trading day. If you are not prepared, you are most likely to engage in emotional trades with poor set ups and terrible risk/reward.
The impact of greed and fear is misunderstood by most traders. Many think that greed makes people to overstay in a certain trade and fear makes them to exit too early, just before it turns in the desired direction. On the contrary: fear makes people to exit too early from their profitable positions and greed makes people stay longer in their losing ones, as the desire to be right more often is stronger than the desire to be profitable.