Good example, but I think the military example better shows how enormous the mental aspect is.

For kids, one could argue that they still have to learn what pain is, just as they have to learn what it is to be physically tired (go on a walk with them until they say they’re too tired to make another step. At that moment, point out there’s an ice cream vendor or a playground a kilometer away, and be amazed).

Kids sometimes also (consciously or unconsciously) play the tired/injured card just to get attention (that probably applies to some grown ups, too)