In order to have freedom, you must be willing to fight for it. The founding fathers understood this concept. They fought and won their freedoms and have tried the best they could to pass those same freedoms, that they fought and won, onto us. However, over time, people will take their freedom for granted and so government will exploit it as an opportunity to take those freedoms away. The theory of the founding fathers was that government is the enemy of freedom, yet the purpose of the country they founded was suppose to be freedom. They recognized that they must restrain government with checks and balances, but they also recognized that these checks and balances could be circumvented. The second amendment, in my view, was placed into the constitution to provide a last resort means for the American people to fight a possible, future government that turned tyrannical and sought to take away their freedoms. Freedom must be earned and fought for, and if you must fight for your freedom, then you must also have the means to do it with. Our independence, our Bill of Rights, turning our principles of freedom into reality was achieved through the force of arms. Thomas Jefferson's views on the second amendment was that, so long as the people were armed, the people will always rule. Take away the means for the people to resist tyranny, then the people no longer rule nor have a hope to be able to control their own destiny, because they are at the mercy of those people who are armed.
That's not to say, that the force of arms is the only way or the first resort. It is an absolute last resort. But, in the end, to keep freedom, you must be willing to fight for it and earn it.