Soulmates or true love are common themes in stories, movies/tv shows and such. I was just wondering if it had any roots from ancient times. I limit my question to these four Mythologies in tags, but other cultures may be accepted as an answer.
The only locus classicus I can think of for the notion of a soul-mate is Aristophanes’ speech in Plato’s Symposium 189c–193d, which does spin a kind of myth to account for it (and for human sexual love more generally, both homo- and heterosexual). But I don't think anyone pretends that it was ever established in communal or tribal oral tradition; it is rather a joke, a burlesque contributed by a comic genius at a party that turned into a competitive round of encomia on Eros. Aristophanes proposes that the human form was once effectively double what it is now, with four each of legs and arms, two faces on one head, and two sets of genitalia, all based around a globular torso. These proto-humans grew to rival the gods in power, so Zeus decreed that they be literally cut down to size, by bisection. The wound was closed (the skin cinched together at the navel), and smoothed over, and became the front (anterior) side of the modern torso, the face being turned to that side also so that we would be forced to contemplate what had been done, as a warning. Sexual love with its embracings (especially front-to-front) is our way of trying to undo this bisection; and the most perfectly matched pairs (whether homo- or heterosexual) are cases where the halves of a single proto-human have somehow found each other. (That the bisection presumably occurred many generations ago, and none of the proto-humans has actually existed since, is a chronological flaw in this theory that Aristophanes does nothing to repair.)
I would argue that this burlesque “myth” itself constitutes evidence that the notion of soul-mate love ran so counter to ancient Greek notions of and attitudes about love as to seem inherently ridiculous. Hence I think we can tentatively infer that the apparent absence of the soul-mate idea from other, more organic Greek myths is no mere accident of (non-)transmission.