Adam and Eve were banned from the garden because of their act of rebellion and disobedience. Had God not banned them, they would have eventually eaten from the Tree of Life, which was also in the garden (Gen. 2:9), and lived forever. Remember, though, that as a punishment for their sin, their bodies were affected with the curse. If they ate from the Tree of Life then they would live forever in a disease ridden body, resulting in their life being an eternal, miserable existence. God banned Adam and Eve to prevent them from an eternity of suffering. This is another act of divine love.