Don’t pretend that you’re above the fray. It’s a good bet that you’ve argued with someone recently over grammar or spelling or regionalisms or some other aspect of language. Perhaps you asserted the correct way to pronounce doge, never mind that the meme only went viral in 2013. If you happen to live in Texas or Georgia, you may have defended the "might could" construction as perfectly fine, thank you. And if you’re from New York or New Jersey, a particularly persuasive outsider may have tricked you into believing that you’re misguided—illogical no less!—for saying "on line" when you really mean "in line."
But why is it that each of us thinks we alone embody the highest authority over how language is supposed to be used? Unless, that is, we've had that notion drummed painfully out of us by high school English teachers, in which case why do we think we are the lowest authority, except for all those other slobs?
One answer lies in what can be called "micro language"—the various structures inside our brain that contain knowledge of what it is to be a speaker of English. Even without other humans to talk to, or any reference guides for that matter, you and every other English speaker has an intuitive sense of what sounds like fluent use of language and what doesn't.
But then we each have our own internal micro language. There are the obvious group differences that we deem "dialects," dictated largely by geography or demographics, including age, race, gender, and education. And there are many more subtle factors, like living in multiple locations at different ages, the more-or-less unique family terms you heard growing up, the topic-specific jargon of the schools and workplaces you've passed through, where you've traveled, any other languages you speak, and even what sorts of interests you have. A doctor who watches a lot of sci-fi will have a slightly different vocabulary than a lawyer who reads a lot of historical fiction, and both of them will have a slightly different vocabulary from their neighbor who's a birdwatcher. Just as no two people live the same life, no two have the same set of linguistic influences. In other words, no two people end up speaking the same language.
Recognizing that each person has an idiosyncratic personal dialect, linguists long ago coined the term idiolect. And it's not just vocabulary; it's everything from how we pronounce certain words to how we put them together to what we imagine they mean. Ever have a disagreement with someone over whether an ambiguously-shaded object was actually blue or green? Congratulations, you've witnessed differences in idiolect. As far as the philosophers are concerned, you're both seeing the same turquoise-y teal object, but you've each drawn a mental line on the continuous color spectrum in slightly different places. Who's right? You're both right, according to your own idiolect. Go ahead and consult some authority, maybe the creators of the Pantone color chart, but really you’re just surveying other people's idiolects.
This is not to suggest that an idiolect is arbitrary. You can't just up and decide one day that you're going to call a pen a frindle, because nobody will know what the heck you’re talking about. Which hints at the concept of a macro language: whatever it is that our mental language representation has in common with other speakers of English that enables us to understand each other (most of the time, anyway).
So what does this common English look like, and where can we find it? There's a tendency, especially in cultures with an extensive written tradition, to say that the official version of English, say, is found in a book somewhere, perhaps the Oxford English Dictionary or various authoritative grammar texts. But a few moments of thought reveals that this cannot possibly be the case. If you're a second-language learner of English who only learns from books—and doesn't talk to any real speakers—you won’t sound nearly as natural as your more sociable fellow learner. Even children of five or six who have barely learned to read See Spot Run, much less tackle a massive reference volume, can construct elaborate, fantastical stories in what's clearly English.
There's a cultural myth that dictionary-makers sit around a table and decide what the new words are going to be, thereby making them officially part of "real English," but as any lexicographer will tell you, that's about the opposite of their actual job. Instead, they comb through massive amounts of existing language, written or recorded, and try to get a sense of what new words and new usages of old words are being produced by sufficiently numerous and diverse groups of people to be of interest to future dictionary users. Dictionaries are inevitably somewhat behind the language as spoken, since people drive linguistic change.
So if it doesn't come from books, where does English at a macro level come from? Your sense of English as a whole is really an abstract combination of all of the idiolects that you've experienced over the course of your life, especially at a young and formative age. The conversations you've had, the books you've read, the television you've watched: all of these give you a sense of what exists out there as possible variants on the English language. The elements that you hear more commonly, or the features that you prefer for whatever reason, are the ones you latch onto as prototypical.
Problems occur when we believe that the macro language and all of the micro languages are or should be the same. You are the absolute authority on English as it exists in your own head, and that's okay. Your intuitions, and even your arbitrary preferences, are real and it's painful to ignore them. Unfortunately, the same goes for everyone else.
But wait, what about "proper" English? Aren't some varieties—idiolects or groups of idiolects—just better than others?
Linguistically speaking? Not at all.
Does it make reading easier and faster if people roughly agree on some writing conventions, rather than everyone picking their own spelling and punctuation haphazardly? Sure. (Although the haphazard method was basically what happened in the Middle Ages, and people didn't seem to mind. It's much easier to learn, for one thing.) And can we still value concise and well-organized speech or writing? Yup. But from a formal perspective, anything that a speaker says intentionally is part of a complex, rule-governed mental system, and they're all equally worthy of interest and respect. So once we’re at a point where we’re all understanding each other, especially in spoken language, how do we form the opinion that isn't is better than ain't, or that ask is better than aks, or that hyperbolic literally is so much worse than the barely-even-hyperbolic-anymore really or truly? Why does Henry Higgins teach Eliza Doolittle to speak like a posh lady, instead of her teaching him to speak like a Cockney flowerseller?
What we think of as "good" English is the English historically spoken by people with the most power. The bumper crop of grammar texts and usage guides that started proliferating in the mid-18th century were part of an attempt by the growing middle class to access economic opportunities that were only available to people who spoke like Henry Higgins. At first, these were primarily a guide to speaking like the upper classes, although, over the years, various arbitrary preferences have found their way in and became crystallized as dogma, so much so that, to quote the linguist Stan Carey, "the aim of these non-rules is to maintain anachronistic shibboleths that allow an in-group to congratulate itself on knowing them."
Can it be a rational decision for the Elizas of the world to modify their idiolect in search of more opportunity? Of course. But at a societal level, it's deeply suspicious that Henry gets to grow up speaking in a way that automatically makes him a better job candidate, while Eliza will have to learn a different dialect than her friends and family if she wants a chance at the same jobs. We don't pick where and how we grow up, and we know that where and how you grow up influences your idiolect, so why is it acceptable to penalize people for something no one has any control over? The answer is simple if your goal is to keep power and economic opportunity in the hands of those who have always had it. We like to think we're more enlightened and less bigoted than our ancestors, but as long as we believe that some idiolects are right and some are wrong, we're not making much progress. "Standard English" is a loose assortment of idiolects like any other dialect, and valuing one over the other is a social construct that has nothing to do with linguistic merit.