The Torah in Parashat Emor (21:11), amidst its discussion of the laws relevant to the Kohen Gadol, commands, “Ve’al Kol Nafshot Met Lo Yabo” – that he may not “come upon” a human corpse. This is understood as a reference to “Tum’at Ohel” – the impurity which one attains by coming under the same roof as a human corpse. The Torah here forbids a Kohen Gadol from entering into a tent or building where a human corpse is present, as he thereby becomes impure. The Kohen Gadol is forbidden to expose himself to “Tum’at Met” (the impurity resulting from contact with a corpse) even in the case of an immediate family member who passed away. He is not permitted to bury the family member or grieve over the body.
The Sefer Ha’hinuch explains that since the Kohanim are designated as an especially holy tribe, who are constantly involved in the service of G-d, they are bidden to distance themselves from Tum’a (impurity). And the Kohen Gadol, the Sefer Ha’hinuch writes, is set apart from the other Kohanim to be “Kodesh Kodashim” – entirely sacred. Although he has a physical body, the Kohen Gadol’s soul is expected to be pure and pristine like the angels in heaven. He is to extend beyond the normal properties of human beings, devoting himself entirely and exclusively to the service of the Almighty. Therefore, the Kohen Gadol is not permitted to mourn and grieve, as he has, in a sense, already separated from his family members by assuming the position of Kohen Gadol and thus rising to an especially high spiritual level. It is almost as though he is not truly living here on earth, and he thus has no need to mourn the loss of family members.
This prohibition applies only to the Kohen Gadol, and therefore, by extension, it applies only in the times of the Bet Ha’mikdash, when there was a Kohen Gadol. A Kohen Gadol who entered a building where there was a human corpse – whether in Eretz Yisrael or the Diaspora – would be liable to Malkut.