If yo google "chemical love" you can find much info on what NRE is from a scientific point of view. The term is a poly term. But the experience happens to all people regardless of mono or poly preferences.
The VERY truncated explanation is: NRE is the "omg we are so perfect for each other i want to be with you all of the time" feelings that arise when u meet someone new.
It is from a biological perspective, designed to promote sex for the purpose of procreation. It is a chemical dump on par with being high on crack-except, your body does it all on its own.
In the mono world, it is what motovates people to rush into commitment and it wearing off (which it always will) people suddenly split up wondering what the fuck happened.
In the poly world its a big press to be with whoever is new and shiny to the neglect of responsibilities to current partners, children, friends, etc.
"Love As Thou Wilt"