One of the central tenets of libertarianism is the idea of voluntary transactions. Why does a libertarian believe this?
In a voluntary transaction, all parties involved are trying to get what’s best for them. They want to maximize their own gains, whether that is real or psychic gain. There are no guarantees that the transaction will be beneficial, but if any party didn’t believe that they would receive suitable benefits, there’s no incentive to engage in the transaction. They would either choose another transaction or simply do nothing.
In short, people will only act if they believe it improves their present situation. They will not act to intentionally harm their situation. If there are no ways to improve their lot, they will not act.
When you introduce the state, every person still operates under this idea of self-interest. The difference, however, is that choices are restricted. Depending on the size of the state, nearly all of the choices may be restricted or only a few. But not only are choices restricted, some choices are made for people.
When either a single person or a group of people makes decisions for others, there’s no way that they’re able to act in the interest of each person they are ruling over. Some people may benefit and be happy, but you’re always bound leave at least some people in a worse position.
And remember, and this may be the most important thing, each ruler is acting in his own self-interest. Since some people do serve others for their own psychic benefit (someone like Mother Theresa), if you look throughout the history of the entire world, you cannot say that about most leaders. Consider how much money politicians end up with these days. Do we have a bunch of Mother Theresa’s running our governments?