Why you shouldn't keep your startup project to yourself, and why you should share the process with the world? It may sound counter-intuitive but...
Feedback can make you change your mind and plans
Others may learn from the value you share, and will, therefore, be eager to share value with you
You are socially under pressure because people expect you to share things and make progress. Therefore you will.
Giving makes you happy and working on a product longterm without feedback or results can make you depressed. Share!
Don't spew out all the secret sauce, but give people a little taste of the soup you're making, and let them know some ingredients.