As the Dow sets new records, the bears go further back in their caves to sulk. By this they mean that they see similarities in today's market that existed in previous periods before stock prices fell. In fact it is different every time. Some of the people that are bearish on the market are technicians.
Technicians look at past data to predict future prices.
Unfortunately it is next to impossible to predict stock prices and technicians end up being the financial world's astrologers. They are great for entertainment value but can be costly to one's financial health. One can go over any period before a market corrected and find similarities. You would, however, find many more differences. If one could profit from using solely past information, there would be many rich technicians.