Some of my ideas were pretty exciting, so usually I'd be bursting to pass on my excitement to someone else - usually my parents. (As any entrepreneur can confirm, coming up with a great new idea is very rewarding) However, I never failed to feel like I lost some intrinsic level of motivation whenever I explained to someone else what I was working on.
It felt like holding a secret only you knew and then letting it out spoiled it. After awhile I learned to contain my enthusiasm and direct it toward making a functioning product before telling anyone... this probably explains why I tend to be much more independent when it comes to my startup ideas...
Working in secrecy makes you feel like you're about to pounce a revolutionary new idea upon the world and take it by storm.