In an effort to praise and understand love, Aristophanes tells a tale in Plato's Symposium of how there were originally three sexes: man, woman, and a union of the two. These primeval creatures were round and had twin sets of arms and legs, and could essentially cart-wheel around. They grew strong and attacked the gods, attempting to scale heaven itself.
Fearing that the humans had grown too strong, Zeus decided to split them down the middle, dividing and diffusing humans into the form we have today. Love, Aristophanes continues, is the reuniting of the original couple:
And when one of them meets with his other half, the actual half of himself, whether he be a lover of youth or a lover of another sort, the pair are lost in an amazement of love and friendship and intimacy, and one will not be out of the other's sight, as I may say, even for a moment: these are the people who pass their whole lives together, and yet they could not explain what they desire of one another.
Aristophanes ancient account mirrors the popular conception of "soul mates," the notion that each person has a unique other half and that once they are reconnected they will be happy and in love. This idea of soul mates is a dominate theme in mass media and it seems to target young adults particularly. After a break up, a common condolence is to tell the brokenhearted, "He wasn't the right one," or "Mr. Right is still out there waiting for you" or some other well intentioned drivel.
It is hard not to recall the now clichéd line from the movie Jerry Maguire: "You complete me." Currently, there is a movie out called How Do You Know? that plays on the assumption that there is only one special person for each of us and a process to verify exactly who it is. (At least this is how it is advertised.)
What these colloquialisms reveal is a kind of teleology, i.e., that there is a specific purpose or end. In this case, the grand finale is finding the one who is specifically intended for you and only you -- your soul mate.
Young folks are often susceptible to this romantic notion early on when parents and role models do not feel comfortable conversing about the alternative--that there may not be anyone perfect for you and you may spend your entire life chasing a phantom created in your youth by such well-intentioned, though superficial, sentiments. That would be an awkward chat.
When talking with youth and trying to challenge these assumptions, be it whether or not soul mates exist, or God exists, or any other big idea, it may be more effective to focus on the reasoning behind the beliefs and let them draw their own conclusions.
The concept of soul mates is passed down through fantastic narratives of lovers finding each other through impossible circumstances, overcoming obstacles, and having an epiphany that they have found the one. The inference from complexity or probability to cosmic kismet is a weak one and should be rejected. The fact that something happened, or failed to happen, does not mean that things couldn't have happened otherwise or that there is a larger cosmic plan at work. Just because you love someone, doesn't mean you couldn't feel that way about someone else, or that your feelings will continue (cf. divorce rate).
If one does believe that soul mates are designed or destined to be together, then issues of free will crop up; would you have a choice not to love your soul mate? What if you had to choose between having a soul mate and free will? (Hollywood: Please do not make this a movie.)
The crux of the soul mate argument seems to come down to a false dichotomy: either soul mates exist or it's all chance. The experience of finding a soul mate suggests that it couldn't be pure chance because what they have is special and anything produced by chance is meaningless, base, and impersonal. If it's not chance, then soul mates must exist.
What isn't captured by this reasoning are the other options that can still respect and account for the experience of soul mates without subscribing to purposeful design. It is possible for things to happen by chance and still be meaningful. Circumstances may involve elements of both chance and predetermination.
The fact that meaningful and passionate relationships exist without such a deterministic structure, that it happens in the face of chance and circumstance, is astounding in its own right. I hear the counterbalance of soul mates in an old episode of Seinfeld, "If there's a woman that can take your presence for more than ten consecutive seconds, you should hold onto her like grim death...which is not far off, by the way."
Excerpt translated by Benjamin Jowett from Collected Works of Plato, 4th Edition, Oxford University Press, 1953.