Greenspan's guarded optimism

In this WSJ article entitled "Seeking Rational Exuberance," Greenspan says we're emerging from a time of irrational exuberance - an overly indulgent investment period. How did this failure in aggregated knowledge happen? Megan McArdle has a nice bulleted list of 'cognitive errors' not the least of which I think is being overly optimistic. Believe me, it's my Achilles' Heel - I would know.