Keep thy heart more than anything that is guarded; for out of it are the issues of life. Proverbs 4:23 (Darby)
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 mouth speaks what the heart is full of. Luke 6:45 (NIV)
I have come to think of the heart as a storehouse. The scripture plainly tells us we can store good in it or we can store evil in it. Philippians 4:8 instructs us to supply our heart with things that are true, honest, just, pure, lovely, virtuous, things of a good report, and things which are praiseworthy. Why? Because the heart is the core of our being, the place where our thoughts originate and then grow into our words and into our deeds. Out of it springs up how we deal with our own lives and how we influence the lives of those around us.
It is plain to see why the Lord admonishes us to guard the heart so diligently. Whatever we store in it is going to flow out of it, whether it be good or whether it be evil. It will reveal our deepest attitudes and beliefs. It is therefore of utmost importance to daily fill our God-given storehouse with good, healthy attitudes and attributes such as are listed in Philippians 4:8. Our lives can then prosper and be well pleasing to Him.