One of the more pervasive and destructive mental tendencies I've seen is that of focusing on what we want instead of what we have. It doesn't seem to make my difference how much we have, we just keep expanding our list of desires, which guarantees we will remain dissatisfied. The mind-set that says "I'll be happy" when this desire is fulfilled is the same mind-set that will repeat itself once taht desire is met.
We want this or that. If we don't get what we want, we keep thingking about all that we don't have and we remain dissatisfied. If we do get waht we want, we simply recreate the same thinking in our new circumstances. So, despite getting what we want, we still remain unhappy. Happiness can't be found when we are yearnng for new desires.