Why We Have the 2nd Amendment

It was in response to this concern:

"[W]hereas, to preserve liberty, it is essential that the whole body of the people always possess arms, and be taught alike, especially when young, how to use them; nor does it follow from this, that all promiscuously must go into actual service on every occasion. The mind that aims at a select militia, must be influenced by a truly anti-republican principle; and when we see many men disposed to practice upon it, whenever they can prevail, no wonder true republicans are for carefully guarding against it."

--Federal Farmer, Antifederalist Letter, No.18

In other words, well regulated militia means that everyone has the right to bear arms.