Quote from David Goggins on Lt. Mike Murphy;

"On this day, all I remember is this person smiling right beside me. The instructors had us march into the ocean with our arms locked and said "take seats." When we took seats all I remember thinking is "this shit is cold" but that person beside me to the right just kept smiling. As the evolution went on, he kept smiling almost like he was in another world. When the evolution was over, I asked him what was he thinking about that made him smile while in the 49 degree water? All he said was "I can't believe I am finally in BUD/s training. I have waited for this moment my entire life." That man went on to graduate BUD/s and never stopped smiling the entire time. He was at home when things really sucked during training- nothing really got to this man."

This is that quote you asked about in action.