Puzzled.

I'm struggling to understand this short-selling business. As far as I can see, it goes like this - if shares in the XYZ company are trading at £10, and Mr. B thinks this price will fall, he can go to Mr. A, who hold shares in XYZ and borrow 100 shares say, from him. He then sells these shares at £10 each, and so gets £1000. If he's right and the price falls to say £8, he then buys 100 XYZ shares for £800 and gives them back to Mr. A together with a small fee for his trouble. He of course has made the best part of £200 profit. Now my problem is this - why on earth would Mr. A play this game - OK he gets his shares back, but they are now worth less than when he lent them, and surely he knows Mr. B's game and that this will be the likely outcome. Why would anyone deliberately damage their own share holdings in this way? Have I missed something?