If, after reading this title, you suppose that this is a patriotic screed you will be excused. You will be wrong, but you will be excused. If I read a title: "Why I am a Christian", or "Why I am a Lesbian", (neither of which would be true) I too would expect some sort of argumentation to follow. It's natural. But what I will write about here is nothing like that. This post concerns only linguistics. Think about it: Both 'Lesbian' and 'Christian' are qualities, while 'America' and 'Lesbos' are places.
I want to explain why we here in the United States have arrogantly grasped for ourselves the quality of being American, when certainly Argentineans, Canadians and all the many peoples in between on this side of our planet would seem to have an equal claim on the term. When traveling to other countries I used to be reluctant to say yes when asked "Are you American?". Often I would just reply "Yes, I'm from the States." I thought it sounded more humble, and I like that quality. But on reflection it is easy to see that Mexico consists of united states, just as we do, so that's not a very good answer either; what makes us think we are The States? So the thing is, there's just no good way around this problem. I have quite given up, and now just reply "Yes, I'm American." Let there be an end to this humility.
The forming of adjectives from nouns is actually a trickier business than one might at first think. If I am from Australia I have a pretty easy job: tack an 'n' on the end and you're done. Simple as pie. You're Australian; easy. If you're from Romania, ditto. Costa Rica, ditto, you are Costa Rican.
Now give a moment's thought to Canada. For some unknown reason they don't follow the pattern. They're not 'Canadan', like every other country name that ends in a, they are Canadian, they have, it seems to me, and somewhat pretentiously, added an extra fillip to the standard formula, which goes to show you that even the seemingly humble Canadians are not entirely without a sense of extravagance—perhaps the French influence.
If one hails from Britain, one becomes British, in the same way that an imp, having a good day, might become impish, or a ripening apple might become reddish, a state of being that implies, hints at a condition, not definitely stating it. The English are rightly credited with understatement. And further, this reticence is often carried to the further extreme of simple truncation as in, "Yes, I'm a Brit." And what are we to make of Japanese, Chinese, and even Taiwanese and Lebanese? The linguistic mind boggles. But the real trouble is this:
What can we call someone from the United States? United Statesian? Doesn't exactly roll around on the tongue does it? USian? I don't think so. How do you pronounce it? So you see the problem.
We needed a term and we took it. Get over it. And of all the places in the western hemisphere, the continents of the Americas, that might have a claim on the term—whether one likes what we do, or dislikes what we do—it would be hard to argue that we're not The Americans. Which will certainly put a shiver down the spines of Canadians and Europeans (who by rights ought to be Europan), many of whom speak pejoratively of American Exceptionalism. What they may not realize is that by assigning us this judgmental phrase they simply confirm the term we have assumed, American, since everyone knows who they're talking about.