Human beings have what is commonly referred to as “energy”. This energy can leave the body and enter into another body where it influences the second body. That is exactly what happens between two human beings in a relationship. The energy of each person leaves their body and enters into and entwines with the body of the other human being in the relationship, or the energy of just one of the human beings in the relationship enters into and entwines with the body of the other human being in the relationship.
The reason “breaking up a relationship” is painful is basically due to exactly what the phrase says. The “breaking up” of the energy linkage between the two human beings.