"Familiarity breeds contempt."
This pertains to relationships in the early courtship phase, relationships that are "exclusive," AND in marraiges, too.
Every knucklehead who seeks exclusivity or who proposes marraige thinks he has found that special woman who is going to somehow defy the 50% divorce rate and fill his heart with a lifetime of love and romance. But the reality is that relationships often fizzle simply because people get bored of each other.
Just the way it is in my opinion.
Human nature.