I can speak about one piece of research:
The research shows that after a baby is born, certain hormones are secreted in both men and women that increases the chances that they stay together, i.e., the hormone attempts to ensure romantic love. Evolution did this on purpose, to ensure that both parents are around for the first 3 years of life (I believe the hormone is secreted for roughly 3 years), in order to give the baby the best chance of survival and success (obviously, two parents are better than one parent). Once the hormone is not secreted or rapidly diminished, well, good luck.

Obviously, it doesn't mean the relationship will end, since many couples are together much longer than 3 years. However, having a brain flooded with specific hormones sure does help.