SOF selection is a proving ground, where your physical, mental, and emotional raw material will be laid bare, with nothing to hide behind. It's a place where you voluntarily walk into a brutal challenge to find out if who you think you are matches who you are. 

It's a way of answering a question that few people are willing to ask. 

Mo Anthoine, a British mountaineer, referred to this as "feeding the rat." 

Think of the "rat" as that gnawing part of you that needs challenge, risk, and uncertainty. You feel better when it's been fed. 

As Anthoine put it, "I think it's because there is always a question mark about how you would perform. You have an idea of yourself and it can be quite a shock when you don't come up to your own expectations. If you just tootle along you can think you're a pretty slick bloke until things go wrong and you find you're nothing like you imagined yourself to be. But if you deliberately put yourself in difficult situations, then you get a pretty good idea of how you're going. That's why I like feeding the rat."

Everyone has a rat to feed. 

You only choose how you feed it.