Sefer HaChinuch ספר החינוך

That a high priest not enter the tent of a dead body: That a high priest not enter the tent of a dead body — and even for one of the six commanded dead [relatives], as it is stated (Leviticus 21:11), “And for all dead souls, he shall not come” — meaning to say, he should not enter into the house with them. As the expression, “he shall not come” implies entering [entering a house].

What I wrote above in Mitzvah 266 in this Order (Sefer HaChinukh 263) is from the roots of the commandment — that it is fitting to distance the priests, who are the holy ones who do the service of God, from the matter of impurity. And [regarding] the high priest who is distinguished to be Holy of Holies; though he has a body, his soul always dwells among the elevated servants. Therefore, the Torah is not concerned to ever permit him impurity — even for the relatives which the Torah is concerned about for the ordinary priests, which I wrote about above (Sefer HaChinukh 263), who will pour out their souls in the house of [mourning] because their hearts are heated for their relatives. The high priest, by contrast, is thoroughly bound to loftier pursuits and entirely separate from human nature. He will lose any memory of this fleeting world; he will not weep for the loss of one of his society because he was already apart from them in life. As far as the laws of impurity from the dead and the tent of the dead, I have already spoken some according to my custom in this Order; it is not fitting to repeat it, lest it become burdensome for the reader. (See Mishneh Torah, Laws of Mourning 3.)

And this prohibition is practiced at the time of the [Temple] when there was a High Priest there. And in any place that he may be — even if on some occasion he exited the Land — he is [still forbidden] from entering into the tent of a dead body. And if he transgressed and entered there volitionally — even [if it was] his father or mother [that] were dead in the house — he is lashed.