If there is any one quote outside of the Bible and portions of the Rome Statute, the Universal Declaration of Human Rights, and the International Covenant on Civil and Political Rights that I have gone back to multiple times, it would be this one by C.S. Lewis:
Love anything, and your heart will certainly be wrung and possibly broken. If you want to make sure of keeping it intact, you must give your heart to no one, not even to an animal. Wrap it carefully round with hobbies and little luxuries; avoid all entanglements; lock it up safe in the casket or coffin of your selfishness. But in that casket–safe, dark, motionless, airless–it will change. It will not be broken; it will become unbreakable, impenetrable, irredeemable. The alternative to tragedy, or at least to the risk of tragedy, is damnation.
It speaks for itself.