Why tell anyone?

[The Valkyrie]

Because some people dislike living a lie, even if it is just to comfort family members.
Because we are what we are and want our family and friends to know the real us.
Because we hate being forced to go along with something we don't believe in, something thousands of "closeted" atheists do on a daily basis (including more than a few clergy).

Why can't the religious keep their religious beliefs to themselves?

If your answer to the last is that you have the need to profess your beliefs, then why should only atheists remain silent?

I don't believe that dinosaurs still exist, but I stay silent about it, I don't go around letting everyone know my belief that dinosaurs are now extinct. Why should I feel the need tell people this?

Exactly as NoraBrimstone said above.

I don't have people stopping me on the street to tell me I'd better believe in dinosaurs or I'll be punished eternally for it.
I wasn't forced, as a kid, to go to a strange place once or twice a week to pray to dinosaurs.
We don't have people trying to retard education by pushing their beliefs that dinosaurs still exist into science class.

If you're an atheist and you want to keep it to yourself, that's your business and your choice. Why should the rest of us be silent because it makes the religious uncomfortable yet they still insist on vocalising their beliefs?