things in demand

I put my Amy Winehouse ticket for sale on gumtree and got eight responses so far. My ticket only got delivered this morning, and seeing my ticket (which I had bought happily in july) and the eight suitors made me realize its value and keep it. I remembered that Sex and the City episode when Charlotte loses the baby when her parents decide to keep her.

Why do we need the confirmation of other people's demand, the threat of losing something to appreciate its value?