Robert Robinson was a converted criminal who became a Calvinist minister in 18th-century England. Come Thou Fount of Every Blessing was written in 1758, and the hymn carries the gratitude of a man who knows he didn't earn his redemption and couldn't have purchased it.
The opening is a metaphor of abundance: Come Thou Fount of every blessing, tune my heart to sing Thy praise. The language is specific: a fount is a source that never runs dry. Robinson is not asking God to top him off; he is asking God to be the source from which his praise flows. Tune my heart suggests that the heart left to itself is discordant; but when aligned with God, it sings.
The middle verse moves to gratitude: Here I raise my Ebenezer, hitherto the Lord has helped me. The Ebenezer is a stone of remembrance, and Robinson is setting one. He looks back on his life and sees God's hand in all of it. The final verse is almost a prayer of warning: Prone to wander, Lord, I feel it, prone to leave the God I love. It is the honesty of a man who knows himself, who has been saved from ruin and fears falling again.