Responsibility. There are many examples of this...for instance, you can purchase a car and freely drive to wherever you'd like, but you must be a responsible driver, with a government issued license and follow legally imposed rules that were decided upon by Representatives who were Voted into office.
Since the very basis of this nation's founding was the concept of allowing all citizens to vote, it has been argued (see the Harvard Law Review paper) that in order to ensure that ALL citizens are able to enjoy their freedoms, then ALL citizens should be required to exercise their right to express their opinion on governance through Voting.
We've all heard the term "Responsible Adult" and perceive it to represent a good thing...even though it implies extra effort on such people's part. Let's ALL act like Responsible Adults and accept the Responsibility of Voting.