Holy of Holies

based on Siach Shaul, p. 340-2

The first half of our parasha is dominated by the special laws that apply to the kohanim. The heart of these laws is summed up: "It shall be holy for you, for I am holy, I am Hashem who makes you holy" (Vayikra 21:8). This follows Parashat Kedoshim, where the entire nation is commanded to act in holiness since Hashem sanctified the entire nation (ibid. 19:2). Thus, the kohanim have an even higher level of sanctity and expectations for appropriate behavior than the rest of the holy nation.
The essence of Judaism is separation and sanctification, as the pasuk says at the end of the previous parasha: "You shall be holy … and I separated you from the nations to be for Me" (ibid. 20:26). There is separation based on nature (inherited) and sanctification based on nurture (the actions we are required to perform and refrain from). The two together make us a special group within mankind.
Judaism does not believe in absolute equality, if that includes blurring differences. Distinctions are that which enable the world to develop, and the ability to distinguish is at the root of all wisdom (Berachot 33a). We contrast, in Havdala, between light and darkness, Shabbat and weekday, and between Israel and the nations. Only these distinctions are responsible for the multi-faceted and creative elements of the world. There is a need for a nucleus of sanctity within mankind in order for the latter to function normally.
Within the holy nation, there is a group that stands out for its even higher level of sanctity. It started with Aharon, who was singled out as holy of holies (Divrei Hayamim I:23:13), first himself and continued with his descendants. When Hashem addressed the "kohanim, the sons of Aharon," it relates to their natural selection. But it does not stop at that, as they became responsible for a slew of requirements and prohibitions. These, not nature, are that which serve to fill the kohanim’s lives with sanctity. They must avoid impurity and choose their spouse with extra care and restrictions. If they do so, then "they will be holy."
Just as the existence of Israel as a special nation is not just for themselves but for the whole world, so too the kohanim’s status is for the purpose of the whole nation. That is the reason that the non-kohanim must treat the kohanim with the honor due to sacred people. The existence of kohanim is the guarantee that the whole nation will be sacred. They are needed as teachers and role models (see Malachi 2:7).
A classic example of this arises in our haftara, as the family of Tzadok is singled out as guarding the Temple, apparently both literally and figuratively, as Bnei Yisrael and other families of kohanim faltered spiritually. This is what protected the whole nation and made them worthy of distinction and reward.