Why do we want what we cannot have, and yet discard those things that are readily available to us?

This applies to people, objects, etc. As a race, we seem to always want something that we cannot have…that is beyond our reach, but those things we have, we mostlydisregard. And even more to the point, when we get those things we could not have, we no longer want them at some future point in time. Why is that? Have you ever been the object of some desire to be had and you could not be had… but once you were, you were discarded?!?