text: First of all, it is not my plan, it is Scriptural and based on 2000 years of Church teaching. 
  My take, and this is my take alone, is that, due to the nature of God and the nature He gave us in His image, there are certain actions that harm our souls, weaken them and in some cases kill them. These actions are called sins and God hates them, not because He arbitrarily doesn't like them, but because they harm our souls to the point that when we die, our souls will not be able to withstand the Fire of God's Love (you read about the burning fire of God in scripture and other spiritual writers). God hates these sins because they destroy our souls and He desires for ALL of us to be with Him for all eternity. But due to His own nature which He cannot change, grave sin-which is not subjective or arbitrary-keeps the human beings that He loves so much, from being able to withstand being in His presence when they die.