“Guard your heart” is a good command. That’s because it’s biblical:
Keep your heart with all vigilance, for from it flow the springs of life. (Proverbs 4:23)
In its context, this verse suggests that keeping—or guarding—your heart means to retain wise words and resist wicked desires. But I’m afraid some people—ahem, me, too often—use it to justify being cowardly or cold instead of loving others, because we think that “guard your heart” means “don’t get hurt.”
C. S. Lewis provides the necessary rebuke:
Of all arguments against love none makes so strong an appeal to my nature as “Careful! This might lead you to suffering.”
To my nature, my temperament, yes. Not to my conscience. When I respond to that appeal I seem to myself to be a thousand miles away from Christ. If I am sure of anything I am sure that His teaching was never meant to confirm my congenital preference for safe investments and limited liabilities. . . .
There is no safe investment. To love at all is to be vulnerable. Love anything, and your heart will certainly be wrung and possibly be 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. The only place outside Heaven where you can be perfectly safe from all the dangers and perturbations of love is Hell. (From The Four Loves, as found in The Inspirational Writings of C.S. Lewis, 278-279.)