דבר אל אהרן ואמרת אליו בהעלותך את הנרות…(בהעלותך ח:ב) 

We can explain the “חלישת הדעת” – despondent state of mind of Aaron HaKohen and Hashem’s words of consolation:

The Gemara says (קידושין לא:א) that “גדול מצווה ועושה ממי שאינו מצווה ועושה” – someone who performs a mitzvah when he is commanded to do so is greater than one who performs a mitzvah uncommanded.

The Yismach Moshe writes in his sefer that in essence it is more praiseworthy  to Hashem when one performs a mitzvah on his own will without being commanded to do so.  But the reasoning of the Gemara is that one who is commanded to perform a mitzvah has a greater Yetzer Hara therefore the reward is greater.

Hence, Aharon had despondent thoughts considering that the Nesiyim brought Karbanos (sacrifices) without being commanded to do so and that is worthier to Hashem.

So Hashem consoled him by saying: “שלך גדולה משלהם, שאתה מדליק ומטיב את הנרות” – your task is greater than theirs since you will light the candles.  Since the mitzvah of  lighting the Menorah wasn’t specifically assigned to the Kohanim, any Yid was able to light the Menorah.  Thus, Aharon used to light the Menorah daily, hence, he performed a mitzvah that he wasn’t commanded to do so.