What we can't have

What is it that makes us want most what we can't have and not appreciate what we do have? It's a simple question, and it's an answer itself to many problems. But the answer is too complicated. In "Imp of the Perverse," Poe talks about the human instinct and desire to cause ourselves harm. Or maybe we just can't settle on something that it wasn't a challenge to achieve. I believe it to be the former.