The Man in Black was the Man in Pain.
"This poignant performance of Nine Inch Nail’s, "Hurt" is almost haunting, as it was recorded just prior to Cash’s untimely death. Whether or not a Johnny Cash fan, this performance is powerful and deep with emotion." Source: FastFocus.TV
Johnny Cash knew pain like few other people, he spent his whole life with it after his jaw was broken during dental surgery and reportedly never healed completely after 34 operations.
When asked about the jaw pain, he used to answer: "It’s pretty severe, almost all the time. Except when I’m onstage. I pray for that, and it works. It doesn’t alter or hinder my performance. I don’t take painkillers."
Strangely enough for our litigious society, Johnny Cash never sued the surgeon despite everybody's advice.
Johnny Cash: A Law Unto Himself. Barney Hoskyns, Mojo 36, December 1996.
Johnny Cash from Wikipedia, the free encyclopedia