When Aharon completed the avodah of the eighth day of the milu’im he blessed the nation. What this bracha consisted of is a matter of dispute. Rashi writes that the bracha consisted of birchas kohanim. Ramban disagrees, as the mitzvah and text of birchas kohanim appears only much later in parshas Naso. As we once discussed (see here, here, and here, Shiurim l'Zecher Aba Mari vol 2 p. 215, Shu"T Chasam Sofer O.C. 22), perhaps one can distinguish between the birchas kohanim of Parshas Naso, which is an independent mitzvah, and the birchas kohanim of our parsha, which appears in the context of the avodah and is part and parcel of the process of avodah. Correspondingly, birchas kohanim in our davening has a dual role as an independent mitzvah as well as an inherent part of tefilah itself.
There are a number of unique characteristics of the mitzvah of birchas kohanim. Even though most mitzvos are in force once a child reaches the age of gadlus, bar mitzvah, the Rishonim write that birchas kohanim is different and until nismalei zekano a kohein should not duchan by himself. Why should this mitzvah be different? The Shulchan Aruch further writes that an unmarried kohein should not duchan, ostensibly because a kohein must be in a state of simcha to duchan and one who is unmarried lacks complete simcha (parenthetical mussar haskel is obvious). The Magen Avraham asks: the only time we find a halachic requirement for a kohein to be married is the kohein gadol performing avodah on Yom Kippur; all other forms of avodah do not have such a requirement. Why should birchas kohanim be different? And why do we duchan only on Yom Tov, reserving the mitzvah only for this time of simcha?
The Aruch haShulchan O.C. 128:49 (link) finds an explanation of these anomalies in a halachic reading of a Zohar on our parsha. The Zohar explains that duchaning causes the Shechina to rest on the Jewish people, and cites the pesukim in our parsha, starting with “Dabeir el Aharon v’el banav…” The Aruch haShulchan suggests that the Zohar means that Aharon and his sons are the paradigm for kohanim who are obligated in the mitzvah of birchas kohanim. Just as Aharon and his sons were married (question that has me stumped: Nadav and Avihu?) when they performed birchas kohanim, and just as their performance of birchas kohanim was done at the height of simcha, during the inauguration of the Mishkan, so too must all future expressions of birchas kohanim be performed by married kohanim in a state of simcha.