There's when a phrase circles in my head in a number of ways for any number of days, weeks, or months, and then that eventually becomes a song. There's a slow permeation of the idea and then that leads into a bunch of themes and becomes a bunch of lines.
C. S. Lewis says that fiction is able to sneak past the watchful dragons of religion. It becomes more powerful to speak in poetry. The song goes straight to the heart while the numbers and the math of it will never be able to reach that.