What makes a relationship go "stale"?

I've heard couples say after years of marriage that they lost that "spark" in the relationship, or something like that. They "fall" out of love, so to speak. How does a couple stay together for so long, and then all of a sudden start to lose interest?
What makes a relationship go stale?
What do you think are some reasons for this, and what could be done to prevent a relationship from going "stale"?
What makes a relationship go "stale"?
Post Opinion