Ah, but you guys are forgetting the "domino" effect. They may not have the direct capability of firing a missile at us, but they can do so to another country or one of our allies. Hence, triggering off a situation in which forces the US to retaliate against them.
The only reason North Korea hasn't starved to death is because the United States comes through with humanitarian aid every time they start making threats. North Korea knows they need us more than we, or any other nation in the world (other than Iran), needs them. They have nothing to gain, and everything to lose, by literally biting the hand that feeds them. They will not be sending missile our way. There are about 100 other problems in the world that are more worthy of your worrying time.
My dear, you are more likely to get in a car wreck then get hit with a North Korean bomb. No, it's not impossible, just not on today's schedule of events. Probably not on this decade's schedule either. Try to focus on improving the world just as much as you are able. It will help us all and ease all of our anxieties a bit.
They haven't developed a missile that is strong enough to carry their nuclear bomb this far. Another way to put it is their bombs are all too big for their missiles to carry them this far. It's good to be aware of world events, that makes you smart, but there probably isn't anything N. Korea can do that will now or ever hurt any country in the world other than S. Korea.