When God is God to you, and when you realize by grace that He has showered upon you blessings without number, you will also feel a need to thank him. It is the knowledge that He is our Father for Jesus' sake that emboldens us to come to God. Prayer is coming to God, coming into the fellowship of Him who has drawn us by His own hand. His fellowship we need. You see, prayer is not a luxury.
Prayer is a necessity.
The central blessing that God gives to us through the cross of Jesus Christ is that we desire to come before God. When we draw near to God, we feel a need of communion with our heavenly Father. This is what the prodigal knew. He knew that he must come to God. By the grace of God he knew the character of his father.