My theory anyway....
I read an article a while back about how the experience of being in love stimulated the same parts of the brain as cocaine. They did tests on mice and found that the reward sites of the brain lit up when there was a 50% chance of a reward. It was the not knowing that made it so exciting.
So I was thinking how this related to duality. There is a sort of love hate thing that happens. You understand each other but at the same time they are a complete mystery to you. Your dual can drive you crazy at times. I'm wondering is it this barrier that keeps the relationship exciting? So many obstacles to be overcome, one week you are terrified they are going to leave and the next week you get a rush when they proclaim their love. Without this uncertainty our brains would not get the same high as we would get in a relationship with someone who is very similar to ourselves.
P.S.- I can't seem to find the article and forgive me if I didn't quote it exactly right but the gist of it was that there needs to be uncertainty for the reward sites of the brain to light up. I did find this article that was quite interesting though.