When God was speaking to Moses on Mt. Sinai, God told Moses "You cannot see My face, for man shall not see Me and live (Exodu33:20). The concept is somewhat esoteric but the Rabbis teach that "seeing" God's Face means to be privy to the inner workings of the world and why things have to happen as they do in this physical existence. That is why a person in this world cannot "see" God's Face because to do so would mean that they understand the hidden, spiritual dimensions of the world.