All behaviours come from the heart. Our words come from our heart and we also feel guilty or dirty because of our heart. Proverbs 23:7 clearly says, "For as he thinketh in his heart, so is he."
Our heart is a big component to who we are, and that is why Jesus is so concerned with it. Jesus wants to help you change your heart, to get it in tuned with the Father's. He wants to give you a new heart.
Our conscious mind processes about 2000 transactions per second. On the other hand, our subconscious area, where our heart is, processes 400 billion transactions per second! Imagine what happens when the living word of God goes into our heart. It will really weed out all the other information.