Proverbs 4:23 Guard your heart, for it is the wellspring of life.
On Dictionary.com the word wellspring gave this meaning: a source or supply of anything, esp. when considered inexhaustible: a wellspring of affection.
Our hearts are the wellspring, the inexhaustible supply, of our love, affections, emotions, and feelings. This should be guarded and protected.
If you found an inexhaustible supply of money how closely would you guard this supply? You'd make sure that this supply did not get in the hands for the wrong person. You'd carefully look over the well being of this supply. You'd guard this supply!
What does: Guard your heart look like for you?