Love is a biologically induced emotional bond. Although we cannot know by human experience what another species truly feels, much testing indicates that love occurs in many animal species and most mammals.
It is true that many people expect more from love than it is. This is probably because of its constant glamorization in movies and other media.
Since it is so strongly felt, many people confuse love with magical things such as that it must last for eternity, that they have found their soul mate, that they were made for each other and various other misconceptions.
There are many reasons why nature could have selected for a very strong emotion of love in our species and science may not have discovered them all. I would say that near the top of the list would be "to keep the parents together long enough to protect, to provide for and to educate the children". The effectiveness of this, as you know, is quite variable.
Perhaps more couples remain together for life than those that don't, but if true, it is far from an overwhelming majority. There are many species that are far more successful than humans in remaining paired for life. I suspect that this incomplete or intermittent pairing is the natural way for humans and not a cultural variation.