why do we want what we can't have?

I recently commented on someone else's blog regarding Adam HaRishon and maybe because we are still in the Braishis parshas, it made me think.
Why is it that Hashem created us so that we want what we can't have and that we're not necessarily satisfied and content with what we do have.
Look at Adam, he had the entire world at his disposal. He could eat whatever he wanted. Did he have to davka choose the one (of the two) tree that Hashem said not to eat from?
Look at us. Maybe I'm just speaking for myself, but I have been in numerous situations in my life where I had something right in front of my face and chose to ignore it for something harder or more challenging to obtain. why? why? why?
Logically it makes no sense. Yet, from speaking to friends it seems to be consistent not just with me. If anybody has any thoughts on this, please help enlighten me.