The traditional period between successive human births is four years - which is also the modal period for those marriages which end in divorce today. The biochemistry of love, it seems, evolved as an 'inclusive fitness' strategy of our species. The capacity to love may be universal but its public expression is culturally controlled. For as everyone's personal experience will confirm, it is an explosive emotion. Given its relatively rapid decay, with settled agriculture the evolved instinct for mates to stay together for about four years and then move on to new partners to conceive and rear new young would have been dysfunctional.
Settled agriculture requires settled households.
If households are in permanent flux there could not be settled households on particular parcels of lands. Not surprisingly, most agrarian civilizations sought to curb the explosive primordial emotion which would have destroyed their way of making a living. They have used cultural constraints to curb this dangerous hominid tendency by relying on arranged marriages, infant betrothal and the like, restricting romantic passion to relationships outside marriage. The reason for this western exceptionalism goes back to the earliest period of the Christian Church, as we have seen. But the Church also had to find a way to prevent the social chaos which would have ensued if the romantic passion its greed had unleashed as the basis for marriage had been allowed to run its course in what remained a settled agrarian civilization.