The thing about telling everyone you're writing a novel is that you actually have to do it.
A wise friend who occasionally makes jewelry pointed this out when I suggested bartering some website design. Once you put it out there, you don't just have the promise to yourself anymore. Suddenly you have to answer to anyone who happens to ask.
This is good, on the one hand. Having lots of people to answer to keeps me from getting stagnant. I can brag about the days I write ten pages (rare but exhilirating), commiserate when the muse is off gorging on the last of the Kettle chips and is in no mood to put out. I hate having to make excuses for myself when I fritter away my few writing hours. I feel not only the weight of my own disappointment, but the weight of all of those I imagine are counting on me to finish what I've started.
And there's the downside - feeling the weight. Which is one reason why I don't talk about my subject except with those very, very close to me (as in, I know what you sound like snoring), or my writing group, without whose sage advice this project would sink before it had a chance to float.
Another is that once I tell the story (by blabbing or otherwise), it's already out there. I don't need to write it anymore, because it exists. Writing would be superfluous. I learned this the hard way, letting the passion of conceiving a new story dribble out my mouth instead of my pen and losing a lot of great material that way. I've talked through two fantasy novels, a middle grade novel, countless articles and short stories before I realized that I needed to keep my ideas sacred, let them simmer in my mind and heart, pouring them out on the page rather than into the air to drift away.
I'm a bit of a blabber by nature, so it hasn't been easy to retrain myself. I still have to reign my mouth in at my writing group, where I often blurt out my newest ideas. Thankfully, it's less dangerous to set those words adrift when I have a safe circle of colleagues to help corral them back to where they belong - on the page, and moving forward.