Do you think while writing?
Or do you think through the theme and plot, and then assemble words on screen? It could be a matter of habit. Sometimes, a good idea can excite us enough to begin writing. For example, this very post.
The ideal case appears to be that of thinking first and then putting thoughts on paper. Thoughts that you have debated several times, and seen them with differing perspectives. It certainly rekuires great discipline.
However, some writers do think while writing. New thoughts enter us in the very process of typing words out — related connections spring up, and memories of events that had hitherto remained buried pull themselves up.
My own habit is to think while I write. Occasionally I do manage to ideate an entire write up — that happens usually when I can find a humorous twist to the plot.