All Kohanim are vested with Kedushas Kehunah - Sanctity of the Priesthood - wherever they are. This special status is generated from the Kohanim's innate connection with Avodah (Temple Service). The defilement of Tumas Mes is an affront to Kedushas Kehunah.
Although we know that God is the source of all life as well as death, the special occurrence of Hashra'as ha-Shechinah - Manifestation of God's Presence - is associated with life. The dynamic immanence of the Shechinah is a force of life. That is why the sick who stood at Sinai at Mattan Torah (the Giving of the Torah) were healed, as the event of Hashra'as ha-Shechinah brought life. So, too, the generations of Bereshis (Creation) had unusual longevity, as they were closer to the Shechinah and could perceive God's Presence. (See Archive: Bereshis.) The Mesilas Yesharim explains that Eliyahu and Elisha were able to revive the dead due to their acute spiritual attachment to God. Hashem's Presence manifested itself through these prophets to the extent that they could revive the dead, as the life force of the Shechinah was conducted through them.
It is thus understood why Kohanim have special stringencies when it comes to Tumas Mes, for death is the antithesis of Hashra'as ha-Shechinah. The active life force represented by the Shechinah, which is the root of Kedushas Kehunah, is utterly incompatible with death (Tumas Mes). The Torah therefore specially bars Kohanim from Tumas Mes.
With this in mind, perhaps we can understand a few somewhat perplexing issues in Parshas Shmini. Upon the deaths of Nadav and Avihu, Aharon and his sons were instructed by Moshe not to let their hair grow, not to rend their garments, and to partake of the korbonos of the day. Whereas an onen (one who just lost a relative) may not consume korbonos, and a mourner does not cut his hair, it was required that these rules and signs of mourning be overlooked. Why is this so?
Parshas Shmini presents the inauguration of the Mishkan, and Hashra'as ha-Shechinah was its centerpiece, climax and very purpose. It was inconceivable that symbolism of death coexist with the manifestation of the Shechinah, which is a force of life. This is why the Torah made such exceptions to normative rites of mourning regarding the special korbonos of that day, which celebrated Hashra'as ha-Shechinah.
So, too, do we find that the Kohen Gadol can perform Avodah on a permanent basis when he is an onen and that he may not leave the Mikdash to follow a funeral procession (Sanhedrin 18a and 64a), nor may he even contract Tumas Mes for close relatives (see Rambam Hil. Avel 3:6-7.). The rationale behind this is that - unlike a Kohen hedyot (regular Kohen), who need not serve in the Mikdash full-time - the Kohen Gadol is always stationed at the Mikdash and is therefore continuously in the realm of the Shechinah. Thus, association with death is contradictory to his role and identity.
We also find - not coincidentally - that death will disappear in the End of Days. This is because the eschatological era will be one in which God's Presence is immanent and very perceptible, and this all-encompassing Hashra'as ha-Shechinah cannot be manifest alongside death. May it be Hashem's will that this day soon arrive.
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