This just isn't always the case, as others have said. Particularly with interest rates as low as they have been, it's often completely rational to pay off your mortgage as slow as you can.
Also, you might not want all your money tied up in your mortgage: Buying a second home, too much risk for your taste in real estate market, etc.
Also, you might not want all your money tied up in your mortgage: Buying a second home, too much risk for your taste in real estate market, etc.