I always had some uncanny notion of this, and would keep my plans to myself. For some things, it's the way to go (like quitting your job and traveling). Probably b/c you don't need the external input; it only adds noise.
Yet I wouldn't be so quick to discourage sharing and doing what it takes to stay motivated. External motivation is great, I wish I had more of it!
I reckon there is greater glory in stating what you will do and following through ftw... and yummy humble pie for you if you give up or fail.
I think you have a good point on external input noise.
When you announce your plans to people close to you, such as family and friends and coworkers, you start getting input which is not necessarily useful even if their intentions are good.
Moreover, your announcement might make some people uncomfortable to the point where they try to sabotage you, either intentionally or subconsciously.
Yet I wouldn't be so quick to discourage sharing and doing what it takes to stay motivated. External motivation is great, I wish I had more of it!
I reckon there is greater glory in stating what you will do and following through ftw... and yummy humble pie for you if you give up or fail.