This post by autopope is an interesting discussion of vampires and parasitology. Some of the comments made me realise that the sort of behaviour shown in a lot of "Soulmate/Soulmark" fanfic AU stories corresponds very well with parasitic infection; the host carries a parasite that needs to mate with another of its own kind (let's say there are ten different types for the sake of argument, that most people are born infected, that the first parasite into a human host stops any other parasite from taking over except under special circumstances such as the death of the first parasite, and that they can only reproduce once or twice). They obviously need to attract another person carrying the same parasite; one way to do it might be to have the bearers born with marks which show which parasite they're carrying, and attract other humans with the same infection - and yes, the parasite is to an extent controlling the host's behaviour by e.g. releasing endorphins an/or pheromones when a suitable mate is near, pumping up the output of chemicals considerably when mating seems likely.
The end result is that you get people born with weird marks who are programmed to "fall in love" with someone whose marks correspond. To make it more confusing the marks might look very different to humans than to their parasites, so that it looks like there are thousands of different soulmarks when the truth is that only a couple of key features (e.g. the precise colour, length, key angles) are relevant to the parasite, the rest is just random.
You can't quite get to the silliest end of soulmark fiction - people born with the first words their true love says to them pre-tattooed on their skin - from this premise, but you can come disturbingly close.