An epiphany I had:

What is a great motivator? Pain. That is, pain is the stimulus for action and activity.

For example, boredom is a type of pain. Boredom is a good stimulus for us to act. Similarly speaking, when your body aches, or your back or knees hurt, perhaps it is your body signaling that it wants to move and to be active. Thus rather than thinking about pain as a signal for you to stop and do nothing, perhaps pain is a signal for you to move and act.