Musar zu Schemot 33:20

The Torah explains that it is impossible for man, while alive, to see G–d. This is why David described the death of the pious as being precious to G–d (Psalms 116,15). Having all this in mind the Torah had to spell out that even a High Priest such as Aaron could enter the Holy of Holies only on one day of the year, on the Day of Atonement. He had to be warned not to enter at will since he had to realize that even the sacred vestments were only made of materials comparable to the כתנות עור ובשר, materials of the physical world, not the כתנות אור worn by Adam prior to his sin. It is only the nature of the Day of Atonement and the forgiveness which G–d grants on that day which enables us to become comparable to angels on that day.