The narrator realized that people's love is just like gravity.
Explanation:
In the story, the narrator who is a young girl comes across an incident where she sees her parents dancing. That was the time when she realized that people are just like gravity.
Just as gravity brings two objects towards one another, in the same way when two people are in love they share a very special bond. Love is the only thing that connects people. So when the narrator sees her parents she realized love is just like gravity.
