On any given day, I'd rather alphabetize my spice rack while getting a root canal than actually write. Sometimes the hardest thing is just to sit down and start typing. I use "Once Upon a Time" a lot to start a scene or just freewrite for a bit or answer a question (like "What does the kitchen in the scene look like?"). Waiting for inspiration to strike to get your daily words in is a good way to never write a word. Gotta just write.
If the question is more about what inspires me to write a specific story (which is another way of saying "where do you get your ideas?"), it depends. Some years ago, I attended an improv workshop at Washington Romance Writers. I learned an important technique that improvisational actors use to keep the scene going: "Yes, and..."
So, if I get an idea to write about a group of people colonizing Neptune as they sail off in their spaceship, the Star of the Sea, I don't say to my muse, "Are you crazy?" I say, Yes, and...then they go straight on to Pluto or Yes and...then they crash into Jupiter or Yes and...they sail into the rings of Saturn... The idea might be insane but the best way to develop it, to get to a viable, workable idea, that might someday become a story, is to greet it with Yes, and... and keep adding to it.
I have never, personally, found writing critique groups very helpful. But a writing brainstorming group is a lot of fun. I still chat nearly daily with my writing friends over Skype, tossing around ideas. I recommend the practice to anyone wishing to keep their creativity supple.