“...He shall be holy unto you...” (Lev. 21:8)

This means that in order for him to be holy he has to be a priest unto you. In other words, it is up to you to see that he conduct himself in a holy manner. The emphasis on the words “for he is holy" stems from the premise that holiness is something that is acquired through one’s free-willed effort, one does not achieve it because it is forced upon one. How then can the Torah command us to “force” holiness on the priest?

...holiness is...acquired through one’s free-willed effort...

This is why the Torah had to repeat: “for he is holy” that the holiness of the priest is of a different nature than that of ordinary Israelites (if and when they achieve it in some measure). We do not impose holiness on the priest. We are commanded to see to it that he does not lose or abandon the holiness which is his by birth. The Torah underlines this at the end of our verse with the words: “for I, the L-rd, sanctify you.” G‑d allows His Presence to rest on the whole people only through the priests. When there is no priest there is no sacrificial service; when there is no sacrificial service, there is no Sanctuary. When there is no Sanctuary, G‑d’s Presence is not at home amongst us.

[Selected with permission from the five-volume English edition of "Ohr HaChaim: the Torah Commentary of Rabbi Chaim Ben Attar" by Eliyahu Munk.]