In "Notes From Underground", what does Dostoevsky mean when he talks about humanity's "terrifying freedom"?

The author argues at length about the possibility that human life will someday be taken to a level of mathematical equation. In that case, free will will no longer exist because every action of every person could be determined—and predetermined—by graphs or equations. The problem, according to the author, is that people want the ability to make their own decisions and to do things simply because they want to—not because it's required. As an example, he points out that some people might say that a sane man would never wish for anything that wasn't in his best interests—an "advantage", as the author calls it. But he says that's not true. He says that sometimes people simply wish for something that's not what is in their best interest just because it's what they want to do.