If you sell a call, you are letting someone else have the option of buying the stock from you at that price anytime between now and the date you set.
So if the stock is $32, but you sell a call at a strike price of $30, at any point between now and the date, they can buy the stock from you for $30, even if the stock price rises to $40. If instead the stock price falls to $25, they would have no reason to buy it from you for $30 when they can buy it for $25 instead.
I have discovered a truly marvelous proof, which this signature is too narrow to contain.