One of the most well-known investors of the 20th Century, Benjamin Graham, said that "the investor's chief problem—and even his worst enemy—is likely to be himself."
What Graham understood—and modern research is catching up to—is the idea that we all have emotions and biases that affect our decision-making. The innate wiring built to survive pre-modern times can be counterproductive in our modern world, especially when it comes to investing.
Check out this article for a few of the human emotions and biases that can adversely impact sound investment decision-making.