No matter how you follow the markets you are seeing record highs all over. Most popular of course is the Dow Industrial Average. Many choose to use the ETF
Well one thing to consider is that not every sector is performing as well as the indices. Retail, transports, energy, the Dollar, all have been sliding or moving sideways this year. The assumption is that the buying in the broad markets could be fueled by investors moving money from those under performing sectors to the indices. Any good money manager has to justify positions and how can you justify a position in a losing sector when the markets are screaming higher? You can't, so you move money to where the action is.
Another, less influential reason could be the level of short investors betting that the markets will pullback. While there is not a large group of people betting on the market moving lower at this point, we can conclude that anyone who has been, or is short the markets has losses. At points along the way as they admit defeat and close those positions they come to market as a buy order which further fuels the trend.
All in all this market continues to be a good tradeable market for short term traders. The long term investors want to be sure they have their line in the sand for where they plan on taking positions off should there ever be a pullback ever again.