So I Be Written in the Book of Love

"Do not to your neighbor what you would take ill from him." (Pittacus)

"Avoid doing what you would blame others for doing." (Thales)

"What you wish your neighbors to be to you, such be also to them." (Sextus the Pythagorean)

"Do not do to others what would anger you if done to you by others." (Isocrates)

"What thou avoidest suffering thyself seek not to impose on others." (Epictetus)

"Therefore all things whatsoever ye would that men should do to you, do ye even so to them: for this is the law and the prophets." (Matthew 7:12)

It's called the ethic of reciprocity. Karma.