GEN461 Abarbanel describes the entire episode of the sacrifices of Cain and Abel (which led to Cain slaying Abel) as a competition between the two for the affection and approval of God.  Genesis Rabbah 22:7 also depicts the relationship of Cain and Abel as competitive, but gives three different underlying reasons for this rivalry.  The first explanation is that they were arguing over possessions: Cain claimed all the land in the world for himself and Abel claimed all the objects in the world as his own.  But then Cain told Abel to get off of “his” land, and Abel told Cain to give him back “his” clothing that Cain was wearing.  They argued and Cain killed Abel.  Rabbi Joshua disagrees.  He says that they spit the land and possessions amicably, but they were actually competing and arguing about whose property would contain the Holy Temple.  Each claimed the Temple site for himself, they argued and Cain killed Abel.  Yehuda bar Ami says they were arguing about and competing with each other over Eve or the twin sister of Cain and Abel who was also born to Eve, as both Cain and Abel wanted to take her as their wives, until Cain killed Abel.  A modern commentary has explained this Midrash to exemplify all competitions in human history, the prototype of what leaders and nations have always fought over and competed for: land/possession, religions, and sex.  AMJV 73