2. Opportunity Cost
By paying extra money on a mortgage, you're missing out on an opportunity to allocate resources elsewhere — this is your opportunity cost. Directing that money into investments instead, for example, can produce greater returns over time.
You don't need to be an extreme risk-taker to make profitable investments. Creating a diverse portfolio can optimize retirement savings. You can automate the process using passive investment and paying monthly for them. In other words, you can set it and forget it.
Also, if it's within your means, contemplate capitalizing on your 401K contribution. As of 2018, the maximum contribution allowed is $18,500 a year if you're under age 50, and $24,500 if you're 50 or older. Take advantage of the free money offer if your employer matches contributions.