If people get something not everyone else has, it makes them feel special, unique, high status. And because of that they’ll not only like a product or service more, but tell others about it.

Why?
Because telling others makes them look good. Having insider knowledge is social currency.

*Note that making access difficult is different from making it impossible. Sure, getting a reservation at Please Don’t Tell is tough, but if people call enough they should be able to snag a reservation.

-Contagious, p.55