high maintenance

I found this same idea echoed by both R' Baruch Sorotzkin and R' Dov Povarski: at mattan Torah the Jewish nation in its entirety experienced prophecy; we heard the dvar Hashem directly.  What could be greater than that!  But Chazal (Mid Tanchuma) tell us that mattan Torah was only eirusin; hakamas hamishkan was even greater -- it was the nesuin, a time when we came closest to Hashem.  "Byom kalos Moshe l'hakim es ha'mishkan" -- Rashi darshens "kalos" as related to the word kallah.  When the mishkan was completed, Klal Yisrael was like a kallah under the chuppah. 

How can the experience of completing the mishkan top mattan Torah?   

Rav Sorotzkin writes that he is always amazed that despite people staying up all night on Shavuos and devoting themselves to learning with such intensity, the feeling doesn't last.  The chassidishe seforim writes that right after Shavuos we have parshas Naso, the longest parsha, the parsha with the most midrashim, the most Zohar, because we exit Shavuos with a bang, with an intensity of focus on Torah that expresses itself in this massive outpouring.  But things don't remain that way.  The parshiyos will come down in length, things revert to the mean.  So too in life, things revert all too soon to the old routine, and oftentimes there is even a slacking off after the chag.  Why does the high of mattan Torah prove so short lived?  

The answer to both questions is that the feelings you have when dating, or during engagement, when you see your kallah once a week or once every few days, may seem so intense that you think nothing can top them, but those feelings ultimately will wane.  It takes the permanence of marriage, the day in and day out of living with someone over time, to really cement a relationship.  The high of mattan Torah is transitory; it takes incorporating it into a home, a mishkan, to make something that endures.  The building of the mishkan, a permanent home trumps the high of the moment of mattan Torah.

Ramban asks why is it that the nazir has to bring a korban chatas after he finishes his nezirus.  What's the cheit -- the nazir successfully completed his term!?

 וטעם החטאת שיקריב הנזיר ביום מלאת ימי נזרו לא נתפרש. ועל דרך הפשט: כי האיש הזה חוטא נפשו במלאת הנזירות, כי הוא עתה נזור מקדושתו ועבודת השם, וראוי היה לו שיזיר לעולם ויעמוד כל ימיו נזיר וקדוש לאלהיו, כענין שכתוב: ואקים מבניכם לנביאים ומבחוריכם לנזירים (עמוס ב׳:י״א) – השוה אותו הכתוב לנביא. וכדכתיב: כל ימי נזרו קדוש הוא לי״י (במדבר ו׳:ח׳), והנה הוא צריך כפרה בשובו להטמא בתאוות העולם.

Ramban answers that the cheit is in not maintaining that level.  A one time spiritual high, no matter how great, that is transient and does not get incorporated into one's regular routine, is like the kallah that has a wonderful engagement but never makes it to the chuppah to build the bayis neeman around those feelings.

Meshech Chochma has a wonderful observation: this korbanos of chatas, olah, and shelamim that the nazir offers is exactly the  korbanos (albeit different amounts and animals) that the nesiim brought during chanukas ha'mishkan.  

 ולכן קרבנותיו המה קרבנות הנשיאים בחנוך המזבח, שלב נשבר הוא טוב מכל הקרבנות, כדאמר סנהדרין מ״ג, והנזירות הוא חנוך לשבירת הרוח והלב המתאוה, לכן מביא קרבנות חנוך

Nezirus is the training period; it has to be followed by a life lived that incorporates the ideals that were learned.   The nazir needs a "chanukas ha'mishkan" to give that new level of kedusha a permanent home so it remains more than just a fleeting experience.