“He had cast out of heaven his dim star; it had fallen, and its track was lost in the darkness of night. It would never return to the sky again, because life was given only once and never came a second time. If he could have turned back the days and years of the past, he would have replaced the falsity with truth, the idleness with work, the boredom with happiness; he would have given back purity to those whom he had robbed of it. He would have found God and goodness, but that was as impossible as to put back the fallen star into the sky, and because it was impossible he was in despair.” The Duel

Someone who's on top of the world isn't much of an observer: happy people are poor psychologists. But someone who's troubled about something is on the alert. The perceived threat sharpens his senses - he takes in more than he usually does. The Post Office Girl 

Variant: Death hangs over thee. While thou livest, while it is in thy power, be good. Book IV

A man can live and be healthy without killing animals for food; therefore, if he eats meat, he participates in taking animal life merely for the sake of his appetite. And to act so is immoral. On Civil Disobedience