I have been researching the phenomenon of being in love and have read around the topic quite a bit ...........?

You're wrong, from the time you "fall in love" on, you can feel like that for almost 2 years, but LOVING someone can last forever. What you mentioned applies only to that state of getting erflies in your stomach and things like that, actual love can come after that state is over and then it can last forever, maybe not. That's why so many couples stay together foever, they love their partners, they're not only in love.