For those who have been trained by it, no discipline seems pleasant at the time, but painful.
Whether you first came upon this message in the New Testament (Hebrews 12:11), in the medicine music of Bobby McFerrin, or in the actions of a demanding teacher, the words ring true.
Some kinds of pain are OK. Some pain is what you need to endure, in order to get to your destination.
Bobby treats it as a mantra, because it needs to be a mantra. The results might be pleasant, but the discipline is painful. Suffer the discipline, suffer the pain, and trust that it will get you where you want to go. Repeat the mantra so you can keep walking through the pain.