[jeffbee]

In another kind of market I can just enter the market. If the price of chairs goes bananas because someone has decided to buy and hoard all the chairs, I can make chairs, therefore increasing supply and restoring a normal price.

In the stock market the number of shares of each issue is fixed, so that doesn't work. This is why the short side is important for price finding. They inject supply into markets even when the long side is trying to manipulate the price up.