John 7:38

He who believes in Me (who adheres to, trusts in, and relies on Me), as the Scripture has said, From his innermost being will flow continually rivers of living waters. 

When we believe and trust in Jesus and His teachings, we fill our innermost being with living water. By its trust in and connection to God, our spirit finds fulfillment.   Fulfillment keeps us satisfied, willing, and able to stay on track with our faith. The flesh is never satisfied without a connection to God, nor can it be.