even Moshe is human

Chazal tell us that opening day of the Mishkan was a tremendous day of simcha, as great as the moment of creation.  Why then, asks Ohr haChaim, does the parsha begin with the word "vayehi," which usually connotes suffering?  He answers:

ולצד שהפציר בשליחות פרעה חרה אפו בו ביום ההוא ועזבו מהיות כהן לעולם, וכשהגיע יום הפקידה נצטער על אשר פרח ממנו עוז תפארת כהונת עולם, והגם כי עצם מאוד בצדקות ובענוה, הלא כל לב חי מרגיש ובפרט בדבר מצוה יקרת הערך ומופלאה כזאת

Even though Moshe reacted with joy when Hashem told him that his brother Aharon was going to be kohen gadol, even Moshe Rabeinu is still flesh and blood and cannot escape a feeling of pain and remorse.  That's the tzaar reflected in the word "vayehi."  

The baalei mussar would eat this vort up.  No one should think they are immune from tempation, from the danger of sin.  

The Baal haTurim writes that because Moshe argued with Hashem for 7 days when Hashem appeared to him by the sneh and he did not want to accept the role of being the go'el of Klal Yisrael, therefore he served only 7 days in the Mishkan and then had to turn the job over to Aharon.  

Of course it's not the seven days of service which are the punishment; it's the taking away of the job.  The 7 days Moshe got to serve only whet his appetite to continue in the position of kohen gadol, serving Hashem in that way.  Worse than not getting the job is thinking for seven days that you have the job in hand only to have it snatched away.

Why should Moshe be punished for his refusal?  The reason why he demurred is because וכל זה שלא היה רוצה ליטול גדולה על אהרן אחיו, שהיה גדול הימנו ונביא היה (Rashi Shmos 4:10), he did not want to steal the spotlight from his brother Aharon.  You would think that Moshe would get a reward for his refusal, not a punihsment!  And where is the midah k'neged middah?  (See R' Chaim Elazari's Nesivei Chaim)

I think the Ohr haChaim provides us with a solution.  Ain hachi nami, had Moshe truly been concerned only for the sake of his brother for those seven days at the sneh, he would be deserving of the greatest reward.  But even a Moshe Rabeinu is flesh and blood.  Even Moshe Rabeinu cannot say with certainty that his refusal was 100% rooted in that l'shem shamayim with no other concern.  

How do we know?  Hashem's answer is "Vayehi ba'yom ha'shemini," with the word "vayehi" connoting tzaar, showing that even the selfless Moshe cannot 100% completely escape the same thoughts and feelings anyone who is human will experience.  

That same infinitesimal amount of self interest that showed itself here was the same infinitesimal amount of self interest that also played a role in Moshe's refusal at the sneh, and the two therefore go hand in hand.