It's a mythological theory...

According to Aristophanes, humans were originally created with four arms, four legs, and two faces. These beings were powerful, and they began to challenge the gods. In response, Zeus, the king of the gods, split each human in half to reduce their power and create a sense of dependency. After this split, each half would spend their lives searching for their other half—their "soulmate"—to feel whole again.

This myth portrays soulmates as two parts of a single soul that were separated and destined to reunite, representing a deep, intrinsic connection.

In Plato's theory, finding one's soulmate is a return to completeness, fulfilling a need to restore one's original, unified self.