What if it Rains

Setting up the full sound equipment for a choir and band to perform outside, for instance in a park, takes an extensive amount of effort. Between speakers, wires, and every small technical detail, by completion it's been several hours of labor, lots of sweating, and a good amount of tactician's effort—how things need to be positioned to sound the best, where they'll be out of the line of vision (but still effective), and the wires that need to be set and draped to avoid a rats nest of tangle. All of this, and what if it rains? You have to pack up and clear out quickly, to save the equipment. Even if the band only played for five minutes, rain doesn't make the process worthless—but it certainly feels that way.

Sometimes you spend a long time on a piece, working very hard and putting your best into it.  Then something goes wrong; someone doesn't like it and 'they' only have negative things to say about it.

That doesn't invalidate it. It is always worth it to write.