This happens because "moms and pops" are piling in, in excitement...but the large institutions are holding back because they're cautious and not convinced. http://www.zerohedge.com/news/2017-10-19/52-weak-high-stocks
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.