Is love a destiny or coincidence in Buddhism?

There are many examples, a couple wants to be together but break up after long-term dating, they want to get married but they never get married. A person wants to get close with another person and vice versa but they can never get together no matter how hard they try, etc. In Buddhism, our life is not destined so how are we supposed to understand above situation? What makes someone has the affinity to become someone's parents or children for, for example, two lifetimes? What power determine the meeting? Is the affinity merely automatically created from the person we normally associate with in daily life?