From The Most Important Thing:
The desire for more, the fear of missing out, the tendency to compare against others, the influence of the crowd and the dream of the sure thing— these factors are near universal. Thus they have a profound collective impact on most investors and most markets. The result is mistakes, and those mistakes are frequent, widespread and recurring.
Inefficiencies—mispricings, misperceptions, mistakes that other people make—provide potential opportunities for superior performance. Exploiting them is, in fact, the only road to consistent outperformance. To distinguish yourself from the others, you need to be on the right side of those mistakes.
Why do mistakes occur? Because investing is an action undertaken by human beings, most of whom are at the mercy of their psyches and emotions. Many people possess the intellect needed to analyze data, but far fewer are able to look more deeply into things and withstand the powerful influence of psychology. To say this another way, many people will reach similar cognitive conclusions from their analysis, but what they do with those conclusions varies all over the lot because psychology influences them differently. The biggest investing errors come not from factors that are informational or analytical, but from those that are psychological.