Going to BUD/S wasn’t my first experience in Special Operations training, and I had already learned to spot the posers and those who’d soon quit, and this guy was one of them. I knew I needed to separate myself from him.

We all form and maintain relationships in our lives for a variety of reasons—they make us feel comfortable, complacent, competitive, strong. For Navy SEALs, at the top of the list is the desire to grow and excel. That’s why elite units and high performers tend to stick together. And that’s why the answer to that age-old mom question “If your friends jumped off a cliff, would you?” is...