I know. I know. This is a gross oversimplification. And yes, I am quite possibly over-exaggerating to the point of misrepresentation. (For example: conservatives don't really fear immigrants as they do illegal immigrants.) But the overall emphasis is clear: while the other two parties are fearful of many issues, we libertarians only have one fear: government.
Now yes, you could argue that "government" could be broken down into various fears: taxes, overregulation, censorship, surveillance, brutality, etc. But in the end, it all barrels down to one issue: government corruption, cronyism, and overall malfeasance. (Okay, make that three things!)
As long as the government is minding its own business and fulfilling its solemn duty to protect and serve the population--that is, ensuring that nobody is being killed, raped, assaulted, defrauded, or stolen from--then we libertarians have nothing to fear; otherwise, we most certainly do.
While the other parties are busy instilling fear over minor things that really shouldn't raise concern, libertarians are concerned about the only real thing we have to fear: government tyranny.