There is an old phrase, "The Seven year itch"
And there is a lot of truth in it.
Human Males are generally larger and stronger than Human females.
In nature, this means only one thing and natural monogamous "Mating for life" is not it. (Nor Monogamy of any sort for that matter)
Monogamy for life is a social construct rather than a biological one. There is natural programming for a Human Male to care for a Human Female while the children are helpless infants, beyond that. If there is no genuine friendship to keep the relationship going, then a drifting apart is entirely natural. 5-7 years is a typical period for this to take place. A continuous stream of children at regular intervals can drag out this process, but many relationships fall apart once the last fledgling leaves the nest
And in any case, before the 20th century, Women often didn't generally live that long anyway. Death in childbirth while still quite young was common up until relatively recently. so even Monogamous relationships were a myth. It was in many cases more a case of serial monogamy at, say, ten years a pop (Or even less!) just like today really but with no need for divorce lawyers
Also, it is often surprising that couples who have lived together quite happily for some time break up shortly after actually getting married. I guess this is down the subtle manner in which the nature of the relationship changes and an inability to adapt to that.
In many ways, the modern world is deeply unnatural and beneith the surface society really has not yet adapted to it.