If it's a goal where completion is obvious, like "I'm going to go back to college", then telling your friends would be a public committment that puts you under pressure to follow through.
Pressure to follow through doesn't always mean you'll follow through; sometimes, it means you won't, specifically because of the pressure!
Some people rebel against pressure, even self-applied pressure. Other people crack under it. And some link the negative affect of the pressure to the tasks they need to perform, conditioning themselves to not work on the task at all.
I see the last kind most often, but the others happen too. And I imagine there might be other ways to use pressure to fail, that I haven't seen yet.
All the books say to tell people when you stop smoking - to allow for peer pressure effects. I think that way is more common.
A blog post by Derek Sivers links to evidence that stating one's goals makes one less likely to accomplish them.
Excerpt:
Link: Shut up! Announcing your plans makes you less motivated to accomplish them.