I am a multiple time world champion breaker (ie breaking boards and bricks), and I know for a fact that pain is a state mind. Through mental training, one can shut out any signal that the body is giving it. I have broken many, many times, and at first at hurts, but then you learn how to shut out the pain. Recently I was at a tournament in New Orleans, where I was defending my title. I broke two bricks with my hand, and my hand got stuck between the bottom brick and the top brick. they both were falling, and the bottom brick hit the floor, and then the top brick slammed onto my hand. My entire middle finger was smashed. It was incredibly painful, but, I blocked out the pain, and got my finger bandaged.
I also believe that pain is weakness leaving the body. through physical training, you build up an indurance. I agree with one of the previous posts, that said something along the lines of, "out of shape people are always the ones complaining after doing something physical".