Happiness

I'm reading this book right now, and it talk about happiness in a very describing way, I think..

It says..

"There's a difference between success and happiness. Success is to get what you want, but happiness is to be glad for what you get. Which means that the concept of happiness includes to be happy without some of the things you wish you had. "

So why are we always craving the things we don't have? When it's being without them and appreciating what we have, that makes us happy :)