Referring to http://en.wikipedia.org/wiki/Option_(finance), for a short call:
If the stock price increases over the exercise price by more than the amount of the premium, the short will lose money, with the potential loss unlimited.
Assume the current stock price is $100, and I hold one stock. Then I sell a call option with a strike price of $100, expiring one year later, for a premium of $10.
So even if the stock price is $1000 a year later, I sell the stock at $100. I still earn the 10 bucks, so why am I going to have a loss? Even an unlimited loss? Is it about opportunity cost? I think it's unreasonable.