Pope Benedict XVI's resignation is a reminder of just how tough leadership really is, says Nancy F. Koehn, a professor and historian. Neither CEOs nor spiritual leaders are supermen, and it's important that they admit that to themselves. "I don't think this is primarily about age. I think it's really about energy and enthusiasm and a kind of physical, moral, intellectual, and emotional verve -- an appetite," she says. "It's something that every leader is responsible for maintaining and feeding."