A great sinner who has been forgiven of all his sins learns to love greatly the One who pardoned him and adopted him into His family.
He who is forgiven much loves much (Luke 7:47). A selfish person who has been delivered from his love for self and the world and the things of the world becomes a selfless, self-sacrificing person.
A person renewed by the Spirit becomes willing and ready to give up everything he has for the sake of the One who has given him everything in the first place.
A hardened heart that has been renewed by the grace of God becomes a generous heart giving cheerfully to please the God who softens calloused heart.
This has been my story. This is also the story of men like the patriarch Abraham, the king David, the apostle Paul, the Latin Father Augustine, the Reformer Martin Luther, the former slave trader John Newton, the evangelist George Mueller, and many other great and not so great Christians in the previous centuries and the present.
This is the same story of many people whom the Lord God was pleased to redeem from sin and save in Christ by His amazing grace.
To some extent, this is also the story of Israel, the Old Testament people of God. They were stiff-necked people. That's how the Scripture described them in several occasions (Exodus 32:9; 33:3, 5; 34:9; Duet. 9:6, 13; 31:27; 2 Chro. 30:8; Acts 7:51).
But God, in His mercy and grace, delivered them from slavery in Egypt. He saved them from the angel of death, led and protected them through their journey, provided for them in the desert, forgave them over and over of their sins.
Like the prodigal son in Jesus' parable, Israel was a wayward son whom the Father has accepted, forgiven, and restored back to his status. One who reads this parable in the Gospel of Luke and the gospel of God's grace in the whole Bible can't help praising God and thanking Him for His kindness and goodness.
This undeserved grace of God is manifested in sending His Son for the sake of and in the place of sinners dying for their sins and coming back to life for their justification.
That's why one is so constrained to love God in return. He is compelled to serve God with his whole heart, mind, soul, and strength. He does this willingly by the Spirit's power.
A transformed sinner obeys God from the heart. He is constrained by the love of God to live a devoted life for this God who saved him is the same God who calls and enables him to a life of holiness and obedience.