The voice of reasoning for shorting is this:
If you can borrow money to invest in stocks (i.e. you are buying more stock than what you can naturally afford) and create an "artificial" amount of demand for a stock through leverage...
then...
It is a natural counter-balance to the falsifying impact of leverage, that one has the option to short-sell a stock. It is as close to an "opposite" of leverage that you can get.
There is risk on both sides.
