No Self

וכל אדם לא יהיה באוהל מועד בבאו לכפר בקודש

The Midrash asks but is not the Kohan Gadol himself a person?  The Midrash answers בשעה שהיה רוח הקודש שרוי עליו היו פניו בוערות כלפידים עליו.  The simple understanding is that the Kohan Gadol was elevated to angelic status and was not considered a person.  In the post, 'Are Angels Men' it was pointed out this seems to disagree with the opinion that even angles are prohibited to enter.  However, the Matnos Kehuna on the Midrash says that is quite difficult for the very previous line in the Midrash by the same opinion, Rav Abahu, says the figure that Shimon Hatzaddik saw coming into the Kodesh Hakadashim with him was Hashem Himself.  Why is he forced to say this?  For he holds the prohibition encompasses angles as well and if so, it must have been Hashem Himslef entering with Shimon Hatzaddik.  So he holds the prohibition applies to angels as well, so what it the answer of the Midrash that the Kohan Gadol was like an angel?  Rav Tzaddok (Pri Tzaddik Erev Yom Kippur #7) explains that the Kohan Gadol lost all form of gashmios upon entering the Kodesh Kadoshim and was elevated even beyond the level of an angel who has a form, albeit very refined.  When the Kohan Gadol entered, he was in total sync with his neshama which is directly connected to Hashem unlike an angel who is a separate being.  

The Mei Hashiloach says וכל אדם means that the Kohan Gadol must remove all vestiges of his אדםness, of his own self, when entering באוהל מועד.  He is not bringing in himself to make an offering to G-d but is there to receive from Hashem that which is totally beyond the human grasp. 

Putting the two Rav Tzadook with the Ishbitzer, the Kohan Gadol is not entering the Holy of Holies due to his great preparations but instead, he is entering into a complete state of negation of self to receive Divine inspiration way beyond one's own grasp.  

There is a discussion on otzer hachama forum as to the geder of this issur with some interesting stories.