I've read an account by a SEAL that emphasized that. He said that some of the guys who made it in his BUDS class puked during the early runs, or were visibly fearful when first confronted by dangerous situations, and some of the guys who were fitness fanatics, or black belts, rang the bell.
This is part of the “selection” process that was used going back to the first spec ops guys like SAS, Rangers, UDT, paratroopers, etc. What the instructor staff is looking for are individuals who, when completely exhausted mentally and physically, reach down into some reservoir they didn’t know they have to find the strength to complete the mission. Most folks will give up. It’s the guys that can adapt and overcome that you want as spec ops people. You cannot mass produce this kind of operator. Less than 1 in 10 people have this kind of drive and determination.