Long ago humans and mythical creatures shared this world together, however humans could never learn to live alongside those who are different from them. They hunted them down, slaughtered them in their hundreds and thousands until almost none remained. If this happened in the modern human world it would be called a genocide however to the humans of that time it was seen as a holy act and the right thing to do.
In time mythical creatures learned to pass as humans to save themselves, spells to make Centaurs look like men and mermaids to walk on land and appear as women became common but no spell was perfect and whenever one of their number was found they would survive a horrible fate at the hands of the humans.
They were facing the end of the multiple races which once made up the magical community. So, they gathered their strongest and brightest magic users to come up with a plan. They where too few in number to fight the humans even with magic, so they did the next best thing. They ran away and formed their own world.
Using ancient magic they made what modern humans would call another dimension, it matched their home one in every way aside from one, no humans. The rivers, forests, seas and skies remained untouched by either human or magical hands. The birds, fish and land creatures knew not of them, it would become their new home.
There was just one problem, their numbers where too small they had been hunted down by the humans for so long that species which once numbered in the hundreds of thousands where down to less then a thousand, some where down into the low hundreds. They knew that this wouldn’t be enough to keep their population going, they would die out one way or another.
Until a new plan was formed, they would take the humans and make them pay for what they did, they wouldn’t kill them no…but turn them into creatures like them and grow the population. So they kept the bridges between their world and the human world open to grab humans.
A man in the woods here, a woman working the fields there. A family in an isolated cabin or a whole ship full of passengers which would be taken in a storm. All these small disappearances and tragedies wouldn’t be noticed by the greater human population but each taken helped them rebuild.
As the centuries moved forward the mythical creatures grew large enough to support themselves, but they had advanced along with the humans. Now the taking of humans was seen as bringing in new blood or rather DNA into their population to avoid inbreeding. This meant going away from taking whole ships or wagon trains full of people, instead a new system was designed one that was seen as better and more ethical.
A certain number of mythical creatures would be allowed to go to the human world, to live as humans, they would live, work, go to school as humans. This was all for them to find love with a human. Once they found love, they would make sure the human was trustworthy and then “bring them home”. Bringing them home was a term for taking them away from the human world and bringing them to the mythical world where they would be transformed into the same creature as their loved one.
This method has soon to be more efficient with a higher rate of acceptance of the change sooner then the old kidnapping methods. So it was done.
The most common age to be sent out are college age creatures or mid to late 20s. Still older candidates are sent out to find love. There have even been cases of mythical creatures who fell in love with humans who already have kids, in these cases the kids themselves are also taken like the partner is. These are riskier but comes with greater reward.