The Harvard economist John Kenneth Galbraith had a famous quote in a book he authored back in 1990: There are two factors contributing to euphoria (bubbles): The first is the extreme brevity of financial memory, and the second is the specious association of money and intelligence.
Ten years is a life time in financial market.
You have Fed QE, CD interest rate of 0.1%, mortgage rate of 3% (8% prior to crash), practically free money after tax and inflation. So, a few early birds, who could, borrowed. The herd then followed. And the cycle started all over again.
You substitute stock market for houses, the effect is the same.
Now you understand why many of the recent ET posts are predicting a crash.
The problem is some of you are really bad at identifying what truly is a bubble.