If currency is deflating at a low, constant rate (say, 2-3%, comparable to the normal rate of inflation), then the interest rate -- in real terms -- must be strictly higher than this. A 0% loan with 2% deflation the equivalent of a 2% real interest rate. But no one will ever lend money in this situation. You take on risk, but have no reward. So the rate of deflation is the minimum real interest rate of any loan in a deflationary currency.

In an inflationary currency, people will lend out money at even a negative real rate of interest and effectively lose money on an investment because the alternative is losing even more money by holding cash. If inflation is 2%, you will consider lending out money at 1% interest (if that is your only choice) because getting 99 cents back on your dollar by investing is better than getting 98 cents back on your dollar by holding cash.

Hence, an inflationary currency encourages both lending and borrowing: lenders lose less real money than holding cash, borrowers pay back less real money than they borrow. A deflationary currency encourages neither: lenders would prefer to hold cash, because it's safer and has decent returns, and borrowers have even higher real interest rates.