On paradigm shifts

Some of the best things I've ever written, speaking as a narcissist, came out completely unexpectedly. Some of the best features I've managed to incorporate into a text were not intended when I started writing it. Some of the most interesting plotlines or simply word conjunctions were written without prior planning and in contrast to the general intention and direction of the piece of writing.
Although I assume the situation is the same/similar for most people, I find this quite interesting. Is it the sheer act and volume of writing that has to be completed before an illumination, stroke of brilliance, clever juxtaposition comes along, or is it that the act of setting out to write itself that brings about an unexpectedly well written passage? More specifically, do you write to achieve these moments, or do these moments prompt the writing, but simply are not discovered before the writing is initiated?

From school essay experience, sometimes really good conclusions can be achieved in the absence of planning and without particular passion for the book. But from personal writing experience, it's the love you have for the story and the words that bring out a particularly powerful aspect of your language, uncharted by neither linguistics nor psychology. I'd call it the language of love if this phrase wasn't utterly pathetic and overused.
I will call it spinal language - when words you yourself write send shivers down your spine, when you get exhilarated, aroused and depressed by your own sentences, when you become so organically involved in your writing that it stops being writing and becomes a transcendental experience involving not so much creation as it does giving birth to a sequence of words.