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?
The Samajivina Sutta, as follows, can hopefully shed some light on this question.


  Once the Blessed One was staying among the Bhaggas in the Deer Park at
  Bhesakala Grove, near Crocodile Haunt. Then early in the morning the
  Blessed One put on his robes and, carrying his bowl and outer robe,
  went to the home of the householder, Nakula's father. On arrival, he
  sat down on a seat made ready. Then Nakula's father &amp; Nakula's mother
  went to the Blessed One and, on arrival, having bowed down to him, sat
  to one side. As they were sitting there, Nakula's father said to the
  Blessed One: "Lord, ever since Nakula's mother as a young girl was
  brought to me [to be my wife] when I was just a young boy, I am not
  conscious of being unfaithful to her even in mind, much less in body.
  We want to see one another not only in the present life but also in
  the life to come."
  
  And Nakula's mother said to the Blessed One: "Lord, ever since I as a
  young girl was brought to Nakula's father [to be his wife] when he was
  just a young boy, I am not conscious of being unfaithful to him even
  in mind, much less in body. We want to see one another not only in the
  present life but also in the life to come."
  
  [The Blessed One said:] "If both husband &amp; wife want to see one
  another not only in the present life but also in the life to come,
  they should be in tune [with each other] in conviction, in tune in
  virtue, in tune in generosity, and in tune in discernment. Then they
  will see one another not only in the present life but also in the life
  to come."
  
  Husband &amp; wife, both of them  having conviction,  being responsive,
    being restrained,   living by the Dhamma,   addressing each other   with
  loving words: they benefit in manifold ways.  To them comes bliss.
  Their enemies are dejected    when both are in tune in virtue. Having
  followed the Dhamma here in this world,   both in tune in precepts &amp;
  practices, they delight in the world of the devas, enjoying the
  pleasures they desire.