I think about eternal love is a christianity from platonism. The Hamlet story or Romeo and Juliet teach you the not simplicity be complexed sentiment of spouse selection.
If strict rationalism are normal strategy for sexual selection the human being never see their eternity for attachment for a favor.
Or some social situation and its social human state, for about the massive population density, too capacity of natural society for spouse selection process, would bring the confusion and disorder random relationship.
But ordinary surplus society and their naturalized humanity for the selective circumstances did make to promise for each other of eternity on the love feeling. If is not so the human being could never get stable family and their familiarity on a living way.
Far east, of Japan, the feeling of eternity can get some member on the pure circumstance for country like social good, (same of England or Britain, megacity and its educational circumstance have been too difficult to do purity human nurture on the sensitivity, gentility).
I saw the bad membership and their customs are West of Japan, specially in Kinki district (So Emperor of Yamato roadside got the bad customs too, geopolitical inevitability).
Often the case with its reason may be from the too much capitals and metropolitan density in the old tradition.
On the social biology says the reproduce strategy makes failure or badness, and diversity on the spouse selective processes, about the human associated too.
