Adam and Eve had two sons ; Cain and Abel, both grew up to be productive men. Cain worked the soil while Abel kept flocks. There came a time when both of them offered sacrifices to God . Cain presented "some of his fruits", while Abel presented "fat portions of some of the firstborns of his flock". It is recorded that God looked with favor upon the offering of Abel or like some bible translations will put it,"the Lord had respect unto Abel and his offering", and as for Cain's sacrifice the Lord wasn't so keen.
It wasn't explicitly stated why the Lord chose the offering of Abel over that of Cain, for me it is quite obvious that Abel's offering had so much value and Cain's seemed like an afterthought. Nonetheless, Cain became angry,he became jealous. Cain ought to have realized his mistakes and asked God for forgiveness, but rather he chose to kill his brother Abel who in fact had no influence over God's decision to accept his offering and reject Cain's. Of course Cain was punished by God, cursed to wander the earth for the rest of his days.
Sometimes in life people become envious and jealous when they see others doing better and just decide to pull them down for no reason. Being jealous portrays a lack of "contentedness"......jealousy comes from being self absorbed , just thinking about your needs and desires with no regard for that of others, wanting all the good things for yourself alone. As Christians when we decide to know the person of Jesus, by the help of the Holy Spirit our minds are renewed, we now want the best for others just as we do for ourselves......jealousy no longer becomes an issue.
Stay Blessed.