One thing that all special units have in common is an aversion to quitting and quitters. The greatest sin, the one unforgivable error is to quit, no matter what the obstacle. Only death can stop a truly determined man. You have to know that the man next to you will not break and run when things get tough.
While not a part of the Special Forces pipeline, the Combat Diver Qualification Course, or CDQC, at the Special Forces Underwater Operations School in Key West, Florida plays a big role in SF culture and lore.
I went through CDQC before it was a seven-week marathon. My hat is off to those guys who took the course after I did. The drills have changed, but the objective remains the same. If you have any trace of claustrophobia or fear of drowning, the clever instructors will find it. They will push that button again and again until you ignore it or quit. There is no try. Every man in my class who failed to graduate quit in pool week. In Ranger School, there were hours and miles between meals where you thought about nothing but how powerfully hunger sucks.
Here is my simple question to you: Would you rather go without a meal for a day, or without a breath for two minutes? The brain has special circuits to warn you that you are dying and cause panic. Key West teaches you to turn them off. That is what killed Chuck.