Genesis 4:1-7 Why ?

Why did God reject Cain's offering?
Verse 7 says, "Gen 4:7  If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door." So God was saying that Cain had not done well. Although the timing or the content of Cain's offering (I have heard both given as explanations for its rejection) may be clues, the ultimate problem was in Cain's heart. God's rejection of the offering served to expose Cain's problem: he did not love his brother, Abel, but was jealous of him.