As a primary-aged youth, I recall being a little confused about why Cain's sacrifice was not accepted. Without understanding why Cain's offering was unacceptable, I wondered if the Lord was being partial to Abel. I remember thinking that fruit and vegetables should be just as suitable as animals, especially since I thought that that was all Cain had. Instead of my first impressions, however, the scriptures and the prophets teach us the reasons and the significance of Cain's rejection by the Lord.
Mormon helps us to understand one reason: "God hath said a man being evil cannot do that which is good; for if he offereth a gift, or prayeth unto God, except he shall do it with real intent it profiteth him nothing. For behold it is not counted unto him for righteousness. For behold, if a man being evil giveth a gift, he doeth it grudgingly; wherefore it is counted unto him the same as if he had retained the gift; wherefore he is counted evil before God". Wherefore, a man being a servant of the devil cannot follow Christ; and if he follow Christ he cannot be a servant of the devil" (Moroni 7:6-8, 11). We read how Cain, possibly following his older brothers and sisters lead (Moses 5:13), "loved Satan more than God" (Moses 5:18) and would not "hearken" to him (Moses 5:16). Alma warned his son Corianton against the possibility of Satan "lead[ing] away your heart again"" (Alma 39:11). The sin is not simply confined to the physical act alone, but also encompasses the mental and spiritual motivation of a love for the devil.