This country was founded upon a simple but revolutionary principle that individuals have inalienable rights and that a government exists only to protect those rights (not determine those rights or vote/take them away when inconvenient). Our constitution is an attempt to outline those rights. (these rights are not privileges granted by a government, but rights of people, held by people, and fought for by groups of people).
Every right comes with a responsibility not only to use that right responsibly but to protect that right for coming generations. The second Amendment exists to ensure that each citizen of this country is able to protect their rights from any threat (abroad or domestic). Each citizen in this country has a constitutional right and responsibility to protect themselves and those under their care. (period)