Our brains light up when we daydream or fantasize about things we want but once we've acquired them, we don't want them anymore. It ceases to exist as a fantasy, thus the joy is gone. It wouldn't be as much of a problem if we were good at counting our blessings and getting joy from what we've acquired, but that's not how most human beings live their lives; we take things for granted. We become desensitized to the objects of our desires once we've acquired them.
Desire must have its objects as perpetually absent. He said its not the "it," that creates more happiness for people in their lives, it’s the fantasy of “it.” This is why some people with less means actually live happier lives; they have more to wish for. Pascal said in so many words, when we're told be careful what we wish for, it’s not because we might be wishing for the wrong things or that we might be disappointed that we don’t get them, but because we’re doomed not to want it once we do.