Thanks for your appreciation and observation VJ. Usually we learn our lessons from difficult situations, so when lesson is learned then situation will change. In your observation you have described two ways for change in situation. There is third way also, behaviour of two persons can change towards each other and usually this is the objective in relationship. It means dynamics of relationship is changed. your question, however, is specific about good relationship where everything is going fine. In this case the reasons of their coming together need to be understood. one reason is a technical reason related to time as time is also a factor in karmic laws. So they have to spend some more time together. Another reason is their learning related to attachment with good time. In such cases after spending good and sufficient time together some unexpected incident may happen to bring sorrows in their lives.

Just one more thing to make it clearer. karmic laws are not static but they are dynamic. So everyone will have different reasons for the events of their lives.