When stocks get toppy and valuations are very high, it doesn't take much to spook the market and for large institutions to hit the sell button. And that's exactly what they did today. http://www.foxbusiness.com/features/2017/08/10/dow-fall-205-points-as-north-korea-tensions-persist.html
Trading seems fun, exciting, exhilarating, etc. so it has a lot of draw to it. And many people won't dig deeper than the euphoric emotion that they feel. However, trading is MUCH harder than investing. And much of the industry is bent towards getting you to trade rather than invest.