Authors love secrets. If you pick up any fiction book off your bookshelf, it's pretty likely that someone in it will have one, or that the whole plotline is based on a secret. Secrets, of course, have to be kept, which gives you the plot driver straight away.
I must admit that as a reader, I'm picky about my secrets. I get can quite annoyed when protagonists keep things from each other when if they just talked the whole thing could be resolved. Of course that would also be an early end to the book.
I suppose one of the criteria for a secret is that it had to have weight. Disclosure has to have significant consequences - life or death ones being the best. Also those that have consequences for people other than the protagonist. Protecting the innocent is another powerful plot driver.
Why, as readers, do we have such a fascination with secrets? Is it innate nosiness? Is it the chance to play detective and try to figure it out? Is it identification - shared experience or the chance to try on an experience that you've never had and can't imagine having. I must admit those books where a group of friends get involved in something terrible as teenagers and then go off to live their lives with that secret behind them sometimes gets me speculating. How would you cope? I'm not sure that I would be able to. There's a common story that if you phoned any group of people and muttered the equivalent of 'Fly - all is discovered' a large proportion would be packed and at the railway station by nightfall.
At the moment I'm unwinding secrets in the WIP, which is why they are on my mind. The majority are not being deliberately hidden, which is why I say unwinding - they are family mysteries, things that my heroine simply didn't know about. I think those are quite common 'secrets'. How often have you read about families discovering a father or grandfather had a distinguished war record that was never mentioned? The advent of popular DNA testing is taking that in a whole different direction too.
In the future I've got an idea for a book that will involve a very big secret, that has been kept for centuries. It's an idea that I've been carrying around for a long time. I hope the time will soon come to write it.
Writers love secrets.