One of my favorite ways to think about language is to compare it to jumping. This may seem strange, but I think it makes a lot of sense. After all, jumping is something that nearly all mature humans can do, provided we have the requisite body parts. And probably, no one taught you how to jump. They may have offered advice or demonstrated their own forms, but you basically learned how to push yourself into the air by observing other people do so and trying it out for yourself. From what we know about child language acquisition, this is roughly analogous to how babies learn to speak.
It's also true of language, as of jumping, that you don't need to mentally review every step of the process before you perform it. We don't typically run through a mental catalog of the muscles we'll need to flex, or the order in which we'll flex them, before launching ourselves off the ground. And we don't need to think about the rules of grammar before opening our mouths to speak. They come naturally, like jumping, and what results is a grammatical utterance just as surely as what happens when we jump is a jump.
You can, of course, think about jumping before you begin to move, and plan out exactly how you're going to do it. You can choose to move in a precise way, more planned out and self-aware than a normal jumping maneuver would require. Language is like that too. If you think about your words carefully before speaking, you can produce a more polished utterance just as surely as you can produce a more precise jump by thinking about the matter first. But an unreflective jump is just as authentic as one with thought before it -- by which I mean, no one would accuse jumping-without-planning of not being a true jump. I don't think anyone would even make the claim that planning out your jumps makes them objectively better than jumping without thinking first.
It is my contention that language is the same way. When we speak without thinking about it, our utterances do not always adhere to the standard notions of grammatical correctness. We speak in run-on sentences and dangling participles, in "misplaced" modifiers and all of the other things that would send an English teacher scurrying for the red pen. If we think carefully beforehand -- as we usually do when we write -- we can generally avoid these items, but they are very present in our unguarded speech. But, like a jump without thought behind it, I see no reason to consider such speech improper, incorrect, or in any way inferior to its more planned-out alternative.
The notions of grammar and grammaticality are still important to a descriptivist, of course, but here too a jumping metaphor can prove useful. For a linguistic descriptivist, investigating which utterances are grammatical and which are not is as pointless and frivolous as considering which jumps qualify as legitimate acts of jumping. We instead take as granted that anyone trying to jump produces a jump, and anyone trying to speak produces a grammatical utterance. The notion of grammar is used to analyze what all of the utterances have in common, just as a general description of jumping could be produced by observing many people's individual acts of it. But descriptivists do not believe that everything is grammatical, just as surely as no one would conclude that any possible motion is a jump. There are some things that simply no one would do when asked to jump, and there are just as many that no one would do when asked to speak.
And this is perhaps the best test of what is a legitimate instance of a particular kind of action. When someone is asked to jump, and attempts to jump, and considers what they have done to be a jump, we should surely accept that they have in fact jumped. And when someone is prompted to speak, and opens their mouth to speak, and considers what they have said to be a meaningful contribution to the conversation (and not just nonsense noises), we should just as surely accept that utterance as grammatical, whether they planned it all out in their head or not.
In the end, language is just like jumping. When we attempt to speak, we speak -- and what comes out is an utterly genuine act.