The shadow is the other half of each of them, the opposite bit of the split self we identify with.
All the partners we meet during life are projections of this other half.
Even if you are not in any one-to-one relationship, you will meet other people who will represent your shadow, such as colleagues, roommates, relatives or friends.
Also no matter how many relationships you have, each of them will portray an aspect of your shadow.
Hence if you are currently have a good time with a partner and you were once with another partner with whom there were many issues, you may project the positive quality of your shadow on the first partner and the negative ones of the second partner.
Yet, all these partners are only there to show you what is hidden within you, so that you can become conscious of what you are projecting.
The task is to ultimately retrieve, transform and integrate whatever you project, and this is what the second round of the zodiac cycle exemplifies.
What makes a relationship sacred is the fact that the partners involved are fully conscious about such a task and experientially acknowledge it as the priority in their rapport.