Genesis 4
3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
4 And Abel, he also brought of the firstlings of his flock and of the fat thereof.
And the LORD had respect unto Abel and to his offering:
5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.
Could it be that Abel's offering being a firstling and being the 'fat' (the best parts, choicest) showed his faith and love in YHWH. While, when Cain offered his 'fruits of the land', there is no mention of these fruits being 'choice' or 'the best', that Cain's faith and love was weaker? That Cain didn't consider that YHWH sees all? Thinking that YHWH wouldn't know that he didn't offer the best/choicest fruits?
Genesis 4
6 And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen?
7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door.
Did Cain sin because he didn't offer the best, because his faith and or love was weak, and he offered what he 'wanted' to, instead of what he should have? Because he chose to please himself, instead of pleasing YHWH?
Netizen, I am going to take a crack at answering your question with a question: When Adam and Eve sinned, what did God do to cover their sin?
Answer (I believe): he slew an innocent animal and gave them a covering. Notice that God could have made a garment of cotton, hemp, or anything. But the fact that blood was shed (for the first time) is significant. The wages of sin is death (Romans 6:23). The act of God was a foreshadow of the great sacrifice that God would one day make of of Himself to save all of us from our sins. The Bible says, "Without the shedding of blood there is no forgiveness of sins" (Hebrews 9:22, Leviticus 17:11)