Okay, that's a good point, but still, where did this concept come from? Someone just decided that we're all free? --The Captain

People got together and said "You there, write down some laws, and if we like them we'll all agree to them". And they did. Freedom was probably picked as one of the things protected by law because people tend to like freedom. Why do governments make laws, and why do we live by them? Well, there are all kinds of theories. Social contract for example.