The scary thing (for me) is that I suspect the answer for a very large number of people is 'yes'. With setting your own rules comes the added burden of being responsible for the results, and plenty of people would like to be able to point to some third party when things go bad.
It's a trick question. :) If someone else had already decided the meaning of your life, you would prefer that, because they would have decided what you preferred, ultimately.
Could they could arrange it so that your purpose was X, and that you preferred that your purpose was not X? I'm not sure if that's actually consistently possible. It's certainly possible for humans to want X and not want to want X, but something with a specified highest goal might not actually be able to hold both.