On Monday February 24th 2020 the Dow dropped over 1000 points in a single day.
Human behavior is driven two forces, fight or flight. When facing a dramatic event we as individuals are forced to REACT to what happens.
When looking at the market, if a person is underinvested and it goes up day after day making new highs, that person may experience FOMO… fear of missing out. They might make the decision to invest based on emotion instead of fundamentals.