Perseverance,, persistence, grit, and determination make a huge difference in the attitude of those that pass the course. How badly do you want it? I don't believe I could stand the cold soak of the pacific ocean for the hours, upon hours, upon hours. I was a slightly built person at that time, and I would have taken an immediate chill leading to hypothermia and would have had to tap out in order to survive.
Survival is critically important as well as the attributes necessary to pass the course, which is not for everyone. Certainly, very tough individuals are formed in the bond of the people that passed.