After 9/11, we had to look at the world differently. After 9/11, we had to recognize that when we saw a threat, we must take it seriously before it comes to hurt us. In the old days we'd see a threat, and we could deal with it if we felt like it or not. But 9/11 changed it all.
Fear and euphoria are dominant forces, and fear is many multiples the size of euphoria. Bubbles go up very slowly as euphoria builds. Then fear hits, and it comes down very sharply. When I started to look at that, I was sort of intellectually shocked. Contagion is the critical phenomenon which causes the thing to fall apart.