The book is about the biology and psychology of creating relationship.
What you created during the "honeymoon effect" was not an accident.
You did create heaven on earth.
During that phase we are usually very present and using our conscious brain
Its only when we slip into our old habits we forget and things feel less exciting..
The interference was the subconsciousness mind and the old programming.
You can rewrite the programming and turn it into a new opportunity.
Understand the mechanisms and you can recreate it.
So when they say its not possible for the honeymoon effect to last,
people just mean they haven't experienced it personally.