Maybe the idea that we can "have it all" is what is making so many people unhappy.

I don't think you can really "have it all." Having it all seems to imply possession, which, in turn, implies that our lives are made complete by the things we can own. I don't believe that our lives are made complete by possessing things; to the contrary, possession only makes us want more of what we don't own or have control over -- it becomes a vicious cycle.