In astronomy, for every two objects in space in orbit, there exists a period of time in which both objects are at their closest they will ever be in orbit. This is known as their "periapsis". In any relationship you have with someone, there exists a periapsis. For example, the periapsis with you and a stranger in an airport is bright and brief, whereas in a deep friendship, there is a period where circumstances will bring you the closest, and while the relationship is still strong, it will not be as close at it was then.
Sometimes, some orbits are so particularly strong that they can pull and change the direction of the orbit of the other planetary object. But regardless, every orbit, relationship. all have their periapsides (pl.).
People you love become part of you — not just metaphorically, but physically. You absorb people into your internal model of the world. Your brain refashions itself around the expectation of their presence. After the breakup with a lover, the death of a friend, or the loss of a parent, the sudden absence represents a major departure from homeostasis. As Kahlil Gibran put it in The Prophet, “And ever has it been that love knows not its own depth until the hour of separation.”
In this way, your brain is like the negative image of everyone you’ve come in contact with. Your lovers, friends, and parents fill in their expected shapes. Just like feeling the waves after you’ve departed the boat, or craving the drug when it’s absent, so your brain calls for the people in your life to be there. When someone moves away, rejects you, or dies, your brain struggles with its thwarted expectations. Slowly, through time, it has to readjust to a world without that person.