Why is it important to guard your heart? The Bible says in the sixth chapter of Luke (NIV), "A good man brings good things out of the good stored up in his heart, and an evil man brings evil things out of the evil stored up in his heart. For the mouths speaks what the heart is full of."
Our heart is our innermost being. It is the core of who we are. What goes in there and who determines that? As children, our parents have a large influence on what gets stored in our hearts, but for most of our lives, we determine what goes in by the decisions we make. The shows we watch, the friends we choose, the lifestyles we adopt, etc., all affect our hearts.
This is why church and fellowship with other believers is critical in, not only our children's growth, but in our own spiritual growth. We guard our hearts when we fill it with the things that please God... good things. Do not fill it with evil and sinful things. Let springs of living water flow from within you out of the abundance of a good heart!
Proverbs 4:23 (TPT) - "So above all, guard the affections of your heart, for they affect all that you are. Pay attention to the welfare of your innermost being, for from there flows the wellspring of life."