Recessions and depressions

There was the great depression in the U.S. back in the 1930s and the 2007-2008 financial crisis.
What is your view on the causes of these massive economic crisis's?
Austrians say it is the low interest rates by the central bank and Keynesians blame it on the animal spirits. Many say it is the combination of many different factors.
What do you think?

Animal spirits?

Dassault Papillon: The "Animal spirits" is the overall consumer confidence and emotions and instincts that guide people.
According to Keynesian economics, if consumer confidence is driven down and a decrease in spending and increase in savings occurs, the economy will spiral into a depression. Aggregate demand (spending) is key to a successful economy. So, GDP would be seen as the determining factor of economic strength.

Bi0Hazard: There are indeed several causes. Chicago school economists would be rather peeved at you ignoring the great stagflation of the 1970s, too.

The basic problem is what factors most affect the supply and demand of money. I think the biggest cause tends to be greed, people trying to grow their markets too fast, and being hit by a supply shock or something. Governments should not seek to grow their economies at any cost, inflating costs in the process.