I think it’s simply due to stress. Most people care about people other than themselves. They worry about the effect of their actions on others. They worry about not living up to their potential. They worry about letting people they care about down. Narcissists by contrast get to live blissfully ignorant low stress lives. Look at Trump. That bastard has not a care in the world. In his mind, he’s already perfect. Notice how the presidency tends to rapidly age people, but it didn’t seem to affect Trump? This is why.