And indeed, they are right. Love is biological bribery. Science has proven that love is a mix of chemistry and emotions. (Or emotions caused by chemistry) that waxes and wanes and goes away over time. And the main reason for it is the perpetuation of the species by a mechanism that is outside of our consciousness and reason.
When love is gone, what is generally left is (sometimes hopefully) a caring, appreciation, and high regard for the object of "love".
And this is exactly why modern divorce rates are so high.
Because marriages were always meant to be a coming together for reasons of survival, children, sharing of responsibilities and a series of common missions.
Instead we have ruined the institution by making the main reason for it to be "love".
So when the love inevitably wanes -- as it naturally does, if the couple has nothing else left (children, shared goals, common missions), why stay together?
And yes, some people might say they have been in love for decades. Which is great! But it rarely happens. Generally love waxes and wanes. Rises and falls. Comes and goes. And those same couples that have been in love for decades will tell you all of the downswings and upswings over the years.
I wish more people knew and understood this. A lot of heartache could be saved. Then again, the world would not be so interesting. Sigh.