Self defense is probably the most commonly deployed argument, and it applies directly to why I'm about to purchase a gun for the first time. I know that in several of the more gun-restricting countries, self defense is not sufficient justification.
Frankly, as paranoid as it seems, I think the best and ultimate justification is as a check against government tyranny. I assume this was in part the reason for its adoption in the first place. Usually I hear two objections to this:
One, that AR-15s have no chance against Abrams tanks, nuclear weapons and aircraft carriers, but I think the Taliban and Vietcong might disagree. Second, that the US government isn't likely to become tyrannical. But I think that's debatable, particularly in the wake of the 2016 election. Partisanship will lead us to do very stupid things.