Anthropologists explain it by examining cultural myths, procreative instincts and mating behavior. None of these are exclusive or necessarily contradictory theories. As with all experiences in the physical, beneath them lies a spiritual root or reality from which these manifestations of desire arise. Desire makes the world go around. It is desire that fuels all life and all creation.
Desire leads to procreation, new life, ambition, all creativity, and every kind of union.
The problem with this is that what we truly seek is unlimited and unconditional, and Romantic love falls short of that ideal. We do long for the unconditional love that most of us have only known from our mothers. Romance by its nature, however, is not unconditional or unlimited. In Romantic love, our feelings are dependent upon what we receive in return. In seeking unconditional love through a limited model of Romantic love, we are never really fulfilled.