Genesis 4:1-5 NIV
Adam made love to his wife Eve, and she became pregnant and gave birth to Cain. She said, “With the help of the Lord I have brought forth a man.”  Later she gave birth to his brother Abel. Now Abel kept flocks, and Cain worked the soil.  In the course of time Cain brought some of the fruits of the soil as an offering to the Lord.  And Abel also brought an offering—fat portions from some of the firstborn of his flock. The Lord looked with favour on Abel and his offering,  but on Cain and his offering he did not look with favour. So Cain was very angry, and his face was downcast.
Abel’s gift comes from the first fruits of his flocks, in offering them he runs the risk that no further animals will follow and compensate. Cain’s gift is not from his first fruits, his gift is an offering, but is easy, at no risk. There is a sense in which he is holding back where as Abel worships wholeheartedly, some might say recklessly.
I will worship, with all of my heart
I will praise you, with all of my strength
I will seek you, all of my days
I will follow, all of your ways
David Ruis, lyrics © Universal Music Publishing Group, Capitol Christian Music Group