From What it Means to be a Libertarian by Charles Murray:
Freedom is first of all our birthright. When all the philosophizing is put aside, what has made me a libertarian is a homely image and the answer to a simple question. The image is of an ordinary human being making an honest living and minding his own business--the kind of person who makes up the vast majority of adults around the world. The question is: What does this person owe the government other than to keep doing what he is doing?
The operative word in that question is government. Our ordinary person owes many things to many people and institutions--to family, friends, community, church, workplace. But an obligation to government is unique. When the government decides you owe it something, that obligation is encoded in law. If you break a law, a representative of the government can compel you by force, at gunpoint if need be, to do what the law demands. The right to initiate the use of physical force, usually called the police power, is what makes government different from all other human institutions.
What should government be permitted to demand of this ordinary person? Very little. Longer and more complicated answers constitute much of the rest of this book. But the short answer gets to the essence of the libertarian position. A person who is making an honest living and minding his own business isn't hurting me. He isn't forcing me to do anything. I as an individual don't have the right to force him to do anything. A hundred of his neighbors acting as a mob don't have that right. The government shouldn't have that right either, except for stringently limited conditions. An adult making an honest living and minding his own business deserves to be left alone to live his life. He deserves to be free.
all italics in original