Thomas Watson wrote,
The first fruit of love is musing of the mind upon God. He who is in love, his thoughts are ever upon the object. He who loves God is ravished and transported with the contemplation of God. “When I awake, I am still with Thee” (Ps 139:18). The thoughts are as travelers in the mind. David’s thoughts kept heaven road, “I am still with Thee.” God is the treasure, and where the treasure is, there is the heart. By this we may test our love for God. What are our thoughts most upon? Can we say we are ravished with delight when we think on God? Have our thoughts got wings? Are they fled aloft? Do we contemplate Christ and glory? Oh, how far are from being lovers of God, who scarcely think of God! “God is not in all his thoughts” (Ps 10:4). A sinner crowds God out of his thoughts. He never thinks of God, unless with horror, as the prisoner thinks of a judge.
Dallas Willard in Renovation of the Heart, pages 106-107.