The item behind glass can seem more desirable simply because it cannot be touched.

Restriction adds a shine that has nothing to do with quality. Behind glass, an object is treated as precious by the arrangement itself, and your mind reads the barrier as evidence of worth before you have examined anything at all. The same item in a basket by the door would not hold you for two seconds. Distance flatters things.

When something suddenly seems necessary, ask whether you would still want it after handling it for ten minutes. Notice how often unavailability is doing the persuading. The same effect operates with people, jobs and invitations that are hard to reach.