If there is a karmic link, it is coming from the sun exactly conjunct north node.
The purpose of this relationship is to define yourselves, to find your identity, to know yourself through the interaction with the other person. In the 12th this works on a very unconscious level, and is very compelling, because it is an important goal in your lifetime, and you feel as though it is destiny driven.
The trine to mars helps activate this mechanism and gives energy to it.
Mercury on the ascendent will make you very comunicative about it, in libra it is oriented towards the meaning of your relationship, the why of being together.
It's basically a very nice composite, well balanced and loving.