Not sure about life's greatest pleasure, but it's certainly been up there on the podium for me for the past 10 years. It's the challenge and satisfaction of simply
creating something (plot, setting, characters, voice, images), spiced up in our case by all that spanking means to us. Your underground chemist analogy, Alan, describes my feelings about this perfectly - I started out mixing heady potions for myself alone, then shared them and found delight in company before easing off the feverish addiction somewhat and dabbling in a bit of business to keep the vintner at bay.
Unlike Alef, I find planning awkward and tedious but the writing an exhilarating rollercoaster. It's surprising and instructive when parts of myself or my life emerge in characters and plots without any conscious intention - patterns that have had to be pointed out to me sometimes by perceptive reader-commenters.
As for judging my own writing, that too is a rollercoaster - on one reading I grin with egotism, on another I wince with embarrassment, though in time these extremes have weathered to a more measured appraisal.
My keyboard's been near-silent for many months now. The tide's out.
But the vintner's banging on my door again, so....

Keep on keeping on, everyone.