That is the opportunity cost of that extra money you are paying. If your mortgage is at 4%, paying extra gives you at most a 4% return. If you can earn more than 4% with that money, paying extra is s poor decision. If you would otherwise just be wasting that extra money on hookers and blow, you'd be better off paying your mortgage.

And this doesn't even address the other point that equity in your house doesn't help with cash flow. If you lose your job, would you rather have a large liquid saving account, or a lower mortgage balance?