Long term stock market return is 10-15%, more towards 15% these days. I can put the money I am going to pay off my mortgage to a S&P500 Index fund and earn 10% return easy. Why do I want to pay off my 5% 30-year fixed? The difference is 10-5 = 5% or higher. For a $300,000 loan that is $15,000/yr that technically govt is paying me to take the loan.