1. It is simply an inalienable right to bear arms, defined in the Second Amendment just as freedom of speech is enshrined in the First. The problem with accepting this argument as the sole explanation is that proponents of gun ownership do not allow for any fettering of that right in the public interest as they implicitly do for most, if not all, other rights, including free speech. That other great American possession, the automobile, kills an equivalent number of people every year as does the gun, and in a similar myriad of ways (although vehicular homicide is considerably less prevalent than gun homicide). The ownership and operation of an automobile is restrained by the very type of laws anathema to proponents of the right to bear arms - they must be registered with the Government, operators must be licensed by the Government, and there are restrictions on where and how they may be operated. Many people in favor of some form of gun control want nothing more than that, but even that much is opposed by the pro-gun lobby, or dies due to lack of support from the public. So, there has to be more to the cultural imperative to be armed than that.
2. An armed citizenry is required to ensure that the Government will not act malevolently against its citizens, as King George III did in the 1770s. This largely theoretical argument fails contemporary scrutiny on two points. The first is purely practical. An armed civilian population, even with the kinds of weapons that it is possible to obtain these days, is simply no match for the modern American military of which, ironically, proponents of this view of gun ownership are usually the most ardent supporters. If you believe that this is the purpose of the right to bear arms, then why vote for governments which increase spending on the very military it would use to oppress you if it wished to do so? Secondly, there does not seem to have been an example of a functioning democracy in the last 200 years that failed because its citizens were unarmed. So where does this notion come from other than from the Revolutionary War, which is so far from the American political situation of today as to be useless as a rationale? I can not see an armed citizenry being necessary in any other democracy in the world, nor being of having any practical effect in America, so this is unlikely to be a rational explanation either.
3. It is the right of citizens to defend themselves and their property, by lethal force if necessary. This has no basis in the Second Amendment (other than the right to bear arms at all, which I have already discussed) but has plenty of judicial case law behind it, including the Castle Doctrine, Stand Your Ground laws and more recently, the Make My Day principle. It is much easier to find compelling practical rationale for the this argument, especially when you read about home invasions and other violent crimes in the news, and of the successful use of guns to stop them. The common argument against it is that America is becoming a less violent place, but there is nothing to say that this isn't because people aren't arming themselves more. The real problem with this argument is that they aren't - gun ownership is in decline just as violent crime is but, inexplicably, as support for the right to bear arms is on the rise. So although this is the most compelling argument for gun ownership in America, it simply isn't being borne out by people's behavior.
So, does this notion of an armed citizenry serve any practical use in American society at all? Is it merely a selfish exercise of a right without regard to its unfettered exercise on the public good? Is it a cultural hangover from the Revolutionary War that is past its time? Is it a case of people fighting harder for a principle that they are less interested in actually exercising?
Is there a compelling argument that I have missed? If there is, I would love to hear it, because something that causes over 30,000 deaths a year without even the practical utility of an SUV just doesn't make sense to me otherwise.