"They shall be holy unto their God, and not profane the name of their God: for the offerings of the LORD made by fire, and the bread of their God, they do offer: therefore they shall be holy." Leviticus 21:10-12 - "And he that is the high priest among his brethren, upon whose head the anointing oil was poured, and that is consecrated to put on the garments, shall not uncover his head, nor rend his clothes; Neither shall he go in to any dead body, nor defile himself for his father, or for his mother; Neither shall he go out of the sanctuary, nor profane the sanctuary of his God; for the crown of the anointing oil of his God is upon him: I am the LORD." - Leviticus 21:6
Moses is being informed concerning the priests of the commandment of Yahweh that the priests, the sons of Aaron, were not to defile themselves by a dead body except for a very close relative. ...the High Priest, as a type of Christ, could not be tainted or defiled by any personal connection to sin since he was in type sinless and pointed forward to the true sinless one, the MessiahDeath is the final outworking of sin but the priests were permitted out of compassion to defile themselves on such occasions. However, the same did not apply to the High Priest.
The reason for the difference between the priests and the High Priest is quite clear; the High Priest, as a type of Christ, could not be tainted or defiled by any personal connection to sin since he was in type sinless and pointed forward to the true sinless one, the Messiah (Hebrew meaning anointed, the Greek equivalent of which is the word Christ). If the High Priest was allowed to be defiled the type would have been broken. The priests, on the other hand, are a type of the "kings and priests" to be made up of the immortalised saints in the Kingdom, who are also priests during the time of their probation and who are touched by personal sin.