Everyone's suffered from Shiny New Idea Syndrome, whether they know it or not. SNIS is what happens when you're just going along, maybe you've got a writing piece of 25-100pgs, and you're committed to it. Then you get a crazy awesome, tantalizing new idea. Like mermaids in space. Or ducks in a court of law. Something that just grips you.
You sit down, write three paragraphs about Starfish (the mermaid who dreams of working for NASA), and then go eat dinner. The next day, you're eager to return to the adventures of Starfish and friends. But when you open up the Word file and try to continue where you left off, you realize you're stumped. Suddenly, it seems like a dooming prospect indeed to document the story of our finned wannabe astronaut.
SNIS happens to the best of us. It also happens to the worst of us, like me. The best metaphor I can think of for it is that it's like having an affair after a solid, ten-year marriage. You can't just slack off a commitment because an appealing new entity presents itself. You have to hold up your end of the deal, and you have to resist temptation.
This is not to say that all ideas stemming from SNIS are bad. As we know, there are no bad ideas, only bad ways of expressing them. (Human falling in love with a vampire? Not a bad idea. Angsty ingrate becoming obsessed with a sparkly vegetarian who likes to play baseball during thunderstorms? Bad way of expressing it.) So how do you pick out the good ideas from the absurd, ducks-in-court ideas?
To paraphrase Jodi Picoult, the good ideas are the ones that you can't stop thinking about. It's easy to forget about Starfish's intergalactic ambitions. But when a really doable, engaging concept grasps you, it'll be the one that won't let go. That's what writing is about: not creating something because you can or because it's easy, but because you have to.
Also, because it's cool to have a job where you wear your pajamas and eat snacks all day.