There's this feeling that happens when you're being pushed to the limit. It's hard to think straight, your mind isn't working properly. I'm sure someone out there could explain it scientifically. I'm guessing it has a lot to do with the bodies oxygen consumption.
It hurts. Your mind is telling your body to stop.
But it feels so good. In those moments your mind is the clearest it will ever be. You earned the feeling. Nothing else matters. You can't think about anything else, the mind won't let you.
Clarity is beautiful.