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Why do shareholders participate in shorting stocks?
When I have stock at my brokerage account, the title is in street name - the brokerage's name and the quantity I own is on the books of the brokerage (insured by SIPC, etc).  The brokerage loans "my" shares to a short seller and is happy to facilitate trades in both directions for commissions (it's a nice trick to get other parties to hold the inventory while you reap income from the churn); by selecting the account I have I don't get to choose to not loan out the shares.