In the course of the detailed instructions for Kohanim which form the opening of Parshat Emor, Hashem instructs regarding the Kohen Gadol, “UMin Hamikdash Lo Yetzei,” “And from the sanctuary he shall not leave” (21:12). Chazal derive from this prohibition that the Kohen Gadol is to perform the Avodah even while he is an Onen, a mourner for a close relative prior to the burial. This is in contrast to the Kohen Hedyot, the regular Kohen, who may not perform the Avodah as an Onen. The simple reason given for this is that while one is at the heightened state of grief of an Onen, his mind is not in control. The sadness that envelopes a person at such a time disqualifies the Kohen Hedyot from performing the service, since he no longer has the proper mindset that is so essential for carrying out his mission properly. The Kohen Gadol, on the other hand, is admonished not to leave the Sanctuary. The Sanctuary must dominate his consciousness; he must reign over his emotions so that he does not even momentarily divorce himself from his lofty mission.
Harav Shneur Kotler Z”L notes the distinction and special demand placed on the Kohen Gadol. Rav Kotler asserts that the capacity to always retain the correct frame of mind crucial for carrying out the priestly service is a prerequisite of Kehunah Gedolah. This is the implied meaning of “from the sanctuary he shall not leave”: the Kohen Gadol must rise from above all worldly events, remaining totally Kadosh and attached to his Avodah. Indeed, as Rav Kotler notes, the ability to remain calm and serene during the most anxious and frightening moments is the hallmark of an Adam HaShaleim, a man who has achieved spiritual and moral perfection. Even as he encounters other people and confronts mundane challenges, he always remains within the confines of the Sanctuary. Indeed, he becomes the embodiment of the Sanctuary, as it permeates his entire essence. Thus, the Kohen Gadol cannot physically leave the Mikdash even as an Onen because by definition, he can never spiritually depart from the Mikdash and the Avodah.