Why should you start working on your goals in secret? Imagine you decided to lose 40 pounds. Would you tell your friends? Will you tell them how motivated you are to go on a diet and start exercising from now on? Think twice before you do.
When you announced a plan, you feel obligated to reach your goal. It instantly turns a fun project into an obligation.
Nobody knows if you fail
If you're not sure it's doable, then take the first steps in secret. If you fail, then nobody will hold you accountable or make fun of you.
Surprise people with something they did not expect
In case you you do accomplish it, people will be admire you for the success they did not see coming because they did not know you even had that goal.
No false feeling of accomplishment
I haven't thought of the third reason myself: As soon as you announce it to your friends, you start feeling closer to the goal already. Watch this TED Talk to see the story behind it.