Commentaire sur Le Lévitique 21:4

לא יטמא בעל בעמיו להחלו This means: he shall not defile himself for his wife if she is really unfitted to be his wife (because he is a כהן) and through whom he consequently becomes profaned as a priest, so long as she remains with him (Sifra, Emor, Section 1 15). The following, therefore, is the literal exposition of the verse: לא יטמא בעל, a husband shall not defile himself for his wife's corpse while it is among its people, which means that there are persons who can bury her so that she is not a מת מצוה (cf. Rashi’s explanation of בעמיו on v. 1). And with reference to what kind of wife do I say this? With reference to such a one who להחלו, i. e. who leads to his being profaned and therefore disqualified from his priestly duties so long as she remains with him (cf. Sifra, Emor, Section 1 15; Yevamot 90b).

לא יטמא בעל בעמיו להחלו, the reason why a priest is not to defile himself through contact with the dead except those of his next of kin, is that also the priest is indeed a בעל בעמיו, a distinguished, highly placed member of his people, it is his task to understand and teach these laws as we know from Maleachi 2,7 “for the lips of the priest are meant to guard knowledge, and Torah and the people will enquire concerning these from his mouth.” This makes him a person to be treated in the manner one treats a politically highly placed person, a king. Seeing he is supposed to be ever ready to serve the King of Kings, in His Temple, it is not appropriate for such a person to desecrate his status even temporarily in order to participate in the burial rites of ordinary people. According to our sages in Sanhedrin 47 the prime purpose of burying and eulogising the dead is to render honour to them. This conflicts with the honour the priests have to accord to G’d on an ongoing basis. However, the Torah exempts the priests from this restriction for burial and eulogising of near relatives from this rule. [some of the last line is my own wording, though it corresponds to the thoughts expressed by the author. Ed.]

לא יטמא בעל בעמיו, no husband among the tribe of the priests is to defile himself ritually