If I go to sleep thinking “what a load of shit, what a wasted evening,” I can usually count on waking up the next morning loving last night’s work. (The opposite is also true).
If I’m doing it as a work-for-hire, and thereby giving up the copyright, there’s a good chance it’ll be my finest work ever. (The opposite is also true).
If I’m writing symphonies over breakfast and conducting them with my spoon–there’s a good chance that I’m cluelessly stealing from something I heard too many times as a reference track two years ago. (i.e., that time I stole an entire guitar solo from Duran Duran and didn’t know it until years later).