"You'll miss it like the dickens, and you'll be changed for the better for the rest of your life," said Mattis, who retired as a four-star Marine general in 2013. "So you'll never regret, but you will have some of the best days of your life and some of the worst days of your life in the U.S. Navy, you know what I mean? That says - that means you're living. That means you're living. That means you're not some p-- sitting on the sidelines, you know what I mean, kind of sitting there saying, 'Well, I should have done something with my life.'"