The three-pound organ in your skull - with its pink consistency of Jell-o - is an alien kind of computational material. It is composed of miniaturized, self-configuring parts, and it vastly outstrips anything we've dreamt of building.
That has to remain the principal reason for doing it, doesn't it? I know it's possible to write for money, and many very good writers have done so. But for me, it has to remain the principal thing that I actually want to do the writing.