The Netziv (25:2) proves from various sources that the donations to the Mishkan were actually forced; there was no choice whether to donate or not. Why was this so – surely voluntary donations would have been better for they actually display the willingness of the donor to give? The idea is that forcing something shows its necessity and importance in life. If one has a choice about whether to do something or not, it shows that the matter in question is not all that important – otherwise one would have to do it regardless of their choice. For example, we have no real meaningful choice over whether we breathe – this is the fabric of our ability to live. Similarly, boys have a bris milah at eight days; why don’t they wait until they are thirteen so they can choose independently to have a bris? For we are showing that a bris is not a ‘choice’ or selection – it is part of the fabric of being Jewish. Furthermore, there’s a deeper level here. Not having the choice to do something shows that there is Someone Above to Whom you are subordinating yourself. If I can choose whether or not to carry out my boss’s orders, he’s not such a strong boss. But if I have no choice in the matter, that’s a sure sign that I am a true subordinate to the boss. The same applies here; having no choice in a spiritual matter reflects the fact that you realise that HaShem is truly above you and cannot be argued with – it is not up to you to decide whether or not you agree with Divine wisdom.
Putting all this together, this is why the Mishkan donations were forced. For HaShem was showing us that we have no choice whether there is a Mishkan or not – it is a major fulcrum of importance in our Jewish lives. Moreover, HaShem was teaching us to subordinate ourselves to Him. Indeed, it is no coincidence that Mattan Torah was forced too; Chazal tell us that HaShem lifted Har Sinai on top of Bnei Yisrael, forcing them to accept the Torah – again conveying that the Torah is not a matter of choice; it is our lifeblood and part and parcel of our Jewish identity.