One time, at a dinner, I asked a famous macroeconomist: "So, what really causes recessions?"
His reply came immediately: "Unexplained shocks to investment."
We really just don't know the answer. Some people — the types who think the market is self-adjusting and wonderful and doesn't need any government help — believe that recessions are a natural, even healthy process. Maybe recessions are responses to changes in the rate of technological progress, or to news of future progress, or even bursts of creative destruction. Others — the people who tend to think the market needs a little helping hand — believe that there's something blocking the market from adjusting to the shocks that buffet it.