I'm not sure I fully understand you.
Love exists conceptually because we rationalize biological changes in our minds and bodies as significant aspects of life. We strive for love because we strive to make our lives significant. Love is how we react based on the causality of nature shaping our brains and the experiences changing the way we think. Two people living in harmony have found meaning. There bodies react positively when they're together. And by that I mean, they trigger each other's pleasure receptors and they make each other healthy. Even without hate, isn't their life meaningful? They live in a flow of being together and being apart and when they come together life is significant. The time apart is just as important because it allows them to replenish their love reserves. Why would they need to hate?