Above all things that have to be guarded, keep or guard your heart. In Hebrew, for it determines the course is translated totsaah (to-tsaw-aw'), which means exit, geographical boundary, or deliverance, source. In other words, as is a man's heart, so is his state of mind. The matters, or issues of life that you let eat at you will consume you, and eventually become your outward expression.
What we think in our heart and mind, we eventually say to others. He is telling us to constantly guard our heart and watch it closely with what we allow to enter into it, because it is the source of life for one's being, both physically and spiritually. The heart is the place - the spiritual soil in which a seed or seeds are planted in. They then sprout and grow until they produce fruit, whether it is good or bad. This is why you must guard your heart. If the heart is pure, your life will be pure; if the heart is corrupt, your life will be corrupt. In comparison, Mathew 15:19 For out of the heart come evil thoughts--murder, adultery, sexual immorality, theft, false testimony, slander. The thoughts of sin are evil and are to be hated. All wicked imaginations, carnal reasoning, lustful desires, and malicious schemes, are included. It is clearly written, these acts of sinful behavior begin and are constructed in the heart of a corrupted being. Every time the Bible defines what it means to be an authentic Christian, it speaks in the language of heart. It's about the heart!
The heart in the first verse is compared with a fountain. If you were to supply that fountain filthy, murky water, that is what comes out. Conversely, if you were to provide purified spring water, your output would be the same pure water. It is similar to the phrase garbage in = garbage out. The key here is to put your mind on the things of God and His scripture will act as a filter, not allowing any wounds or hurts and anything else to cause a seed to plant. Keep focused on the things above not giving anything a chance or a place to grow to create any physical or spiritual problems in our lives.