Scarcity and exclusivity help products catch on by making them seem more desirable. If something is difficult to obtain, people assume that it must be worth the effort. If something is unavailable or sold out, people often infer that lots of other people must like it, and so it must be pretty good. […]

Scarcity and exclusivity boost word of mouth by making people feel like insiders. 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. When people who waited hours in line finally get that new tech gadget, one of the first things they do is show others. Look at me and what I was able to get!

Jonah Berger — Contagious: Why Things Catch On