I'm pretty sure it was Plato who invented the popular "other half" notion of the "soul mate". (I'll let someone else Google that.) I, myself, certainly do not subscribe to the notion that I, or anyone else, is a half of a whole that needs another person to "complete" them. In fact, I think all truly happy and healthy partnerships (whether poly or mono) are as a result of whole people coming together in a mutually enhancing sort of way -- but one without much or any sense of lack. The more one is oriented around lack, the more likely they are to be unhappy. And only other chronically unhappy people are 'happy' to spend a lot of time around chronically unhappy people.
Healthy people experience passing, temporary, acute moments of unhappiness, and generally experience themselves as a whole--not a half. Chronically unhappy people dwell in a sense of incompleteness, insufficiency, lack.... Dwelling there in their minds, so does their heart and "soul".