Fantasies have to be unrealistic.

Fantasies have to be unrealistic because the moment, the second that you get what you seek, you don't, you can't want it anymore. In order to continue to exist, desire must have its objects perpetually absent. It's not the "it" that you want, it's the fantasy of "it." So, desire supports crazy fantasies. This is what Pascal means when he says that we are only truly happy when daydreaming about future happiness. Or why we say the hunt is sweeter than the kill. Or be careful what you wish for. Not because you'll get it, but because you're doomed not to want it once you do. So the lesson of Lacan is, living by your wants will never make you happy. What it means to be fully human is to strive to live by ideas and ideals and not to measure your life by what you've attained in terms of your desires but those small moments of integrity, compassion, rationality, even self-sacrifice. Because in the end, the only way that we can measure the significance of our own lives is by valuing the lives of others (The Life of David Gale).