Hedonic adaptation.

"It’s cruel but true:"

We’re inclined — psychologically and physiologically — to take positive experiences for granted. We move into a beautiful loft. Marry a wonderful partner. Earn our way to the top of our profession. How thrilling! For a time. Then, as if propelled by autonomic forces, our expectations change, multiply or expand and, as they do, we begin to take the new, improved circumstances for granted.

Understanding that might help you avoid making life's worst mistakes — like devaluing your marriage/throwing it away.