BBC: Genesis 4:2-7
And again, she bore his brother Abel. Now Abel was a keeper of sheep, and Cain a worker of the ground. In the course of time Cain brought to the Lord an offering of the fruit of the ground, and Abel also brought of the firstborn of his flock and of their fat portions. And the Lord had regard for Abel and his offering, but for Cain and his offering he had no regard. So Cain was very angry, and his face fell. The Lord said to Cain, “Why are you angry, and why has your face fallen? If you do well, will you not be accepted? And if you do not do well, sin is crouching at the door. Its desire is for you, but you must rule over it.” - Genesis 4:2-7
This is a hard text. It is a clear text that requires a controlled reading. Abel, the second born, is not given a name of promise. And yet this passage concludes with Abel being placed above Cain due to his sacrifice. Why was his sacrifice accepted? The author of Hebrews says his gift was offered by faith (Heb 11:4). Many things can be read into this (e.g. obedience to a commandment, blood sacrifice, no first fruits, etc.). Though application can be derived from reading into the text, the text itself does not attempt to describe why.
The literal meaning of "regard" is to look and gaze upon. The Lord turned His face towards Abel and his offering. This simple description must have manifest itself in a simple way to Cain. In like fashion the drastic verb "to fall" of Cain's demeanor and emotion is of value to God. Even in the rightful rejection of Cain's offering, the Lord still reaches out to Cain.
The Lord's question to Cain is not a true interrogative. Instead the Lord uses it to reflect back on Cain who the responsible party is. Here many may find a proof text refuting Calvinism. This is solely a misunderstand of the doctrine of predestination that has never permitted harm or damage to the free will of man. To refuse to do well, through faith, is to sin. This truth is not only a description of the murder of Abel but of the whole of Cain's decisions.
The desire of sin is to reign. Christ Himself confirms this doctrinal truth (John 8:34). Christ Himself demolished this reign (1 Cor 15:54-57).
*The Bible Blogging Commentary is a slow and simple treading of the Scriptures. No quotes from other theologians and no explicit Greek/Hebrew lessons.*