Why guard your heart? Simply because it is the wellspring of life. (Proverbs 4:23) Wellspring. Meaning source, beginning, supply of something. It is where everything started. Your Emotions, happiness, anger, envy, faithfulness, loyalty, love, passion, your ideas and thoughts. It is where everything started.
Why guard your heart? Simply because out of the abundance of your heart, the mouth speaks. ( Matthew 12:34) What you say, intentionally or unintentionally first felt by your heart. The gossip you spread, comes from the hatred in your heart. The affirmation you said, comes from the appreciation of your heart.
Why guard your heart? Simply because it is the deceitful of all. (Jeremiah 17:9) Since every emotions first felt in your heart, Anybody can feed your with lies that will result to negative emotion.