Expectations

What is it in human nature that causes us, usually normal and carefree individuals, to have, no, to SET expectations? Why on earth do we put ourselves through the misery of hoping, wishing and wanting things to happen, turn out, or come true? It is pure agony. Unnecessary torture.

Here is what happens when we make expectations:
You find something, something that is enticing and new, different from the norms that you are used to. This thing, whether an idea, a person, a job, is so amazing that you cant get it out of your head. Thoughts and images of it are burned into your brain. Then you realize that there is no way this thing that you have found could become yours or turn to work in your favor. maybe its just not in the cards. But you still want it, think about it, let it consume your thoughts and distract you from the normal day to day activities. Then, out of some sort of miracle, this little dream or idea or person or job or whatever it could possibly be shows up, and it's like your dreams have come true.
You are elated! You cant imagine life being any better! But then something happens, that job doesn't work out, dreams and ideas that were once grand don't seem so grand anymore, that person changes. And the expectations you had, the ones that once had you so distracted, they are all just a broken puzzle scattered on the floor.
You want nothing more than to piece everything back together the way it was. You try, but as it seems the most important pieces are missing, or got chewed up by the dog, or ripped up by the lawn mower. Still there, but never able to be what they once were.
Where do you go from here? Do you buy a new puzzle and start all over again? Is it worth it to rebuild the broken pieces?