The writers of the constitution had just come out of an oppressive government and won their rights by fighting back against the oppressive government and decided to make sure that if there was a time wen the tree of liberty had to be watered - the citizens would be armed and step up and form militias to fight back.
The whole point of America is and was Freedom. It's a responsibility that all americans must stand up for. Be it an outside force, an oppressive government, or your home being invaded, it's up to you to stand up and fight.
We've collectively been slapped down by many oppressive laws - the right to keep up in technology (I don't mean computers here), the right to be not spied on or searched illegally, and the right to be free.
Here's a basic litmus test. Will you agree to be searched before boarding a flight, or will you tell the person that you're an American, and you refuse to have your basic civil rights violated? It's ok to answer honestly, I just would like to know since I'd like a tax on folks like you, to fund people like me since we need more ammunition since you aren't stepping up to be ready to fight for your freedom.