I was in a class with a guy who was a swift boat driver. He went to BUD/S, but rang the bell at about 11 weeks. Everyone there was in outstanding physical condition prior to going, but he said it was the mental toughness that made the difference. Suppressing pain, fear, discomfort, etc. He said the thing that got him was the cold. There was no way to get warm and everything was wet and full of sand. Water was always up your nose and the sleep deprivation was insane. After 11 weeks his body just couldn't go any further and he rang the bell.