The "Scarcity Principle" dictates that the more somebody has access to something, the less they will come to value it. It seems to be a biological thing.
When we over-share people get annoyed. If we could never get away from our significant other we'd breed resentment. If we spoil our kids, receiving a gift or present becomes the norm.
But by withholding just enough we breed interest, desire, and fascination.
When Snapchat makes a message that disappears in 24 hours, you want to make sure you see that thing before it's gone forever.
When your product will only be available for 72 hours, you make it more desirable.
When the sofa you're looking at has been sold to someone else, you find that it just happens to be the one you REALLY wanted.
We're after the forbidden fruit, not the fruit that is right beside it and very much available.
It's a funny thing, that "Scarcity Principle" and it's as old as mankind himself.