In Pluto book 2, JWG talks about the various kinds of relationships that a soul will attract- and why those relationships. If you don't have a copy I'd be happy to summarize it here, that would be wonderful practice for me
There is always a reason why any two souls will be in relationship with each other. First the evolutionary and karmic dynamics of each particular soul has to be understood- then based on that, we can move to composite chart and the synastry in order to understand the nature of the relationship itself.
In the end there can be any number of reasons why two souls have come together in whatever way they have. As I understand it, most, if not all relationships have a past life story to it. It's the nature of a soul's journey to be in constant relationship with everything. As it's progressively burning it's separating desires, it's also engaging in relationship- thus creating karma with many other souls based on what internal dynamics are being worked on. All souls are doing this.
JWG writes on page 2:
So we meet again, eh? Yes we do. Many many times. Why do we do this? The answer is very complex, yet can be boiled down to this: Souls evolve over great lengths of time by desiring to experience a variety of necessary experiences, dynamics, behaviors, circumstances, values and beliefs that ultimately allow the Soul to be reunited with the Source of Creation through an exhaustion of all possible experiences that generate or create the appearance of being separated from, or separate from, the Source.
Certain dynamics that dictate the need for specific kinds of experiences generally take more than one life to work through. Because we evolve by focusing on certain dynamics for a period of time in order to focus on other dynamics at another period of time, we tend to keep re-creating circumstantial realities in life after life that reflect those key dynamics. In so doing we create relationships with people whose nature reflects the dynamics that we are working on, or are defined by. Thus we re-meet those people, and they us, for many lives until those dynamics are exhausted and evolved beyond.