Love GET BORN, GROWS, MATURES, DECLINES AND FINALLY IT DIES. Love within a person to a person follows this trend. Some couples are lucky enough that love between them ages with their life. But to majority of the couples, love dies while the couple lives.
When love dies while the couple lives, what you have described here happens automatically. Couples live together without love; love has died off! Some will look it elsewhere, some will cling to loveless relations.
It is our wish and the religions' wish that love between COUPLES is permanent. But reality is that in most COUPLES LOVE IS NOT PERMANENT and is subject to weathering; wear and tear as we go along. What makes love weather so fast between some couples than others is not well understoood, but some believe evolutionary forces are at play!!
May be we are in a transition from organisms in whom love between couples is TEMPORARY to organisms in whom love between them is PERMANENT.
But while we live, we will continue to see what we see! May be in a 100th generation to come all love will be permanent because we belive that permanent love will be PREFERED by natural selection forces.