To a certain degree, the episode of the Eigel HaZohov remains cloaked in mystery. It would be difficult to imagine that a nation would stoop to mere idol worship after achieving the מדרגה, the level, on which they were able to merit Kabbalas HaTorah. Consequently, many Meforshim explain this episode in a more subtle fashion, saying that since the reappearance of Moshe Rabbeinu was somewhat overdue based on the calculation made by the people, they panicked and sought some alternative method through which they could express their religious feelings. This route led them to build and somehow serve the Golden Calf.
No matter how innocent the mistake of making the Eigel might have been, however, it was nevertheless a grave sin, and the Eigel thus had to be destroyed. It should be of no surprise, though, to see that on the heels of this scenario comes the arduous task of building the Mishkan as per Hashem's commandment. This seems to be a display of Hashem's attribute of operating via the approach of מדה כנגד מדה, measure for measure. After showing this incredible desire for a vehicle comprised of gold, Bnai Yisrael were given the mission to make Hashem's "House of Gold." The building itself, however, was not the desired achievement. The goal was to accomplish the construction out of pure effort. The Torah stresses this point several times by using the phrase "כל נדיב לבו", "everyone who is generous of heart" (שמות ל"ה;ה'), and many variations thereof, throughout our Parsha and others which deal with the donations for the building of the Mishkan. This indicates that the donors' motivations had to be pure.
Why does it matter who donates the materials for the construction of the Mishkan? It would seem that the owner's attitudes in some way rub off on his possessions. It would therefore seem logical that the donations of someone with less than an altruistic attitude would be rejected. We would not want someone who is out merely to gain fame, or for some other goal, to play a crucial role in the construction of the Mishkan. Such a person is not "generous of heart;" his contributions are not needed.
Moreover, the Ohr HaChaim, commenting on the first Posuk in our Parsha (שם פסוק א' בד"ה אלה) indicates that Shabbos is introduced here as a תיקון, or repair, for a previous blemish. It would seem that this blemish is the חטא העגל, the sin involving the Golden Calf. In this capacity, Shabbos is an antidote for people's previously shown weaknesses. Indeed, Shabbos contains the message that no matter how earnest and devout one's actions are, they come with limitations. Six days are enough in which to do work. There must then be a suspension of work and dedication, even with positive motivations and for positive goals, through a different track, namely Shabbos. How much more should we enjoy Shabbos in view of the idea that Shabbos and the Mishkan were given to us, at least partially, in order to facilitate control over emotions that previously had no better outlet. Our Parsha actually refers to this notion of control when the people were told not to give any more for the Mishkan because enough had been collected, and the Posuk states "ויכלא העם מהביא," "And the nation stopped from giving" (שם ל"ו:ו'). Rashi (שם) comments on the word "ויכלא," saying that it is "לשון מניעה," an expression of restraint. Obviously, the people wanted to continue giving, but the limit was reached and Moshe sent out the word that enough was collected, and a measure of self-control had to be exercised.
The message to us is clear. In any of the various pursuits in which we engage, it is of paramount importance to keep in mind that the ultimate goal is עבודת הבורא, serving our Creator. The more we toil, the easier it is to be blinded by our own satisfaction and enjoyment. Only through honest dedication and objectivity, and with pure motivations guided by self-control, will we see the true Nachas from our actions.