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Why do shareholders participate in shorting stocks?
There are two primary reasons shares are sold short:  (1) to speculate that a stock's price will decline and (2) to hedge some other related financial exposure. The first is acknowledged by the question.  The second reason may be done for taxes (shorting "against the box" was once permitted for tax purposes), for arbitrage positions such as merger arbitrage and situations when an outright sale of stock is not permitted, such as owning restricted stock such as employer-granted shares. Why would a shareholder lend the investor the shares? The investor loaning his stock out to short-sellers earns interest on those shares that the borrower pays.  It is not unusual for the annualized cost of borrowing stock to be double digits when there is high demand for heavily shorted shares.  This benefit is however not available to all investors.