Medrash Tanchuma explains that Moshe was saddened because, although he delivered all the instructions for the Mishkan, he had no personal involvement in the construction. Hashem understood that Moshe wanted to be part of this great mitzvah. When the workmen couldn't erect the Tabernacle on their own, they brought it to him. Although Moshe was not physically strong enough, Hashem told him to begin building and He would then"lend His hand"and complete the process. Moshe would nevertheless be credited for building the Mishkan based on his deep desire for the mitzvah.
Why was it was necessary for Moshe to erect the Mishkan when he too needed Hashem's help? Hashem could have helped the workmen just as easily. Rabbi Zev Leff, citing the Meshech Chochma, notes that no object or time has intrinsic holiness except that which is conferred upon it by Hashem through the Torah and its teachers. Moshe represented the Torah, and only he could invest the Mishkan with the holiness it needed to serve as a dwelling place for Hashem. In doing so, Hashem taught us that Torah directives are given to us through the sages, and that we cannot sacrifice Torah law to achieve something sacred, whether it's building the BeitHamikdashor preserving the sanctity of Shabbat.
Along these lines, The Sichot Ba'avodat Hashem notes that the intention at the beginning of any activity or time frame will imbue the entire time with that essence. For this reason, we recite kiddush right at the onset on Friday night, so that the entire Shabbat will be invested with sanctity. The early Christians,lehavdil, understood this concept and therefore decided mistakenly to make the first day of the week their"holy day." What they neglected to remember, however, was that we begin our day at night, and we end Shabbat by extending it into the evening, thereby investing the beginning of the following week with sanctity. He explains the adage, "Kol hatchalot kashot," to mean that everything must begin with hard and strong resolve. Before performing any mitzvah, one must first resolve to do the mitzvah properly for the sake of Heaven, so that even if one falters, that initial intention remains in place.
With these two concepts in mind, one can understand why it was necessary for Moshe to erect theMishkan. As the Shaarei Chaim explains, Moshe possessed the qualities necessary to erect the Mishkan for Heaven's sake. He had spiritual and physical strength, as well as the humility to nullify his ego completely. Thus only he could imbue the Mishkan with the proper sanctity from the moment of its inception.
If Moshe's passion to participate in the physical building of the Mishkan was already there, why did Hashem require him to act on this desire? The Chochmat Hamatzpun notes that Bnei Yisrael were commanded to bring the sheep to their homes in front of the Egyptians, tie it to the bedposts, and then to slaughter it. Wasn't their belief in Hashem after so many plagues enough to prove their loyalty to Him? Since intention is so important, why go through such a tedious, dangerous, process? But in Judaism, intention alone is not enough. Just as Bnei Yisrael were required to act on their faith, so Moshe needed to act on his passion before Hashem would help him complete the process.
Rabbi Wolbe writes that Hashem wants us each to build a sanctuary within ourselves so He can dwell there. It is not enough just to believe in God, we must act to create sanctity in our lives by performing the mitzvot with love, intent and focus. The Tiv Hatorah notes that Moshe perceived the task of erecting the Mishkan as daunting. But Hashem demanded that he begin and only then did He help him. So too, we must begin our daily battles with the yetzer hara even if we feel weak, and count on Hashem to help us. For that reason, says Rabbi Rabinowitz, if we have indeed invested all of energy, we should not berate ourselves if we fail. Hashem alone determines our success and will reward us immeasurably for our efforts.