Once we are in love with that person, our interactions make use of the archetype. Other archetypes would include mother, father, infant. It is essentially the same process that causes baby birds to imprint the first animal they see as their mother. Jung would say that they exhibited an archetypal response to the infant. Or consider childless people who treat a pet like a child rather than an animal.
Archetypes are much the same as what are often called instincts.
Keep in mind that Jung started his career working in a lunatic asylum at a time when modern drug-based treatments were not available. There was little that could be done in terms of treatment, but he had ample opportunity to study the ways in which people's minds went wrong. This is what led him to the idea that the basic structures of the mind are the same, but in insane people they operate in inappropriate ways. Thus a stalker might be making use of the spousal archetype, but applying it to a person he never met. You could study the operation of these common archetypes by studying other primates, and we find that there do appear to be built-in instincts for parenting, forming social groups, etc.