I’ve spoken and written about the Honeymoon Effect before. Basically, people feel more excited and happy when getting engaged, and marrying, but after the honeymoon, they become desensitized (accustomed) to what it feels like to be married. When they have adapted to the change, the excitement and happiness boost goes away. There are things you can do to keep that up (mostly focusing on appreciating what you have), but it’s not the point of this article so I won’t go into it further here.