Soulmates
In the year of every teenagers 18th birthday, they will awake in their sleep and unconsciously write a letter to their soulmate. Whomever the letter ends up with, is left with the responsibility of finding the other person and make them fall in love with the sender, until eventually, marriage.
But what happens if someone doesn't have a soulmate? Or what if their soulmate died? Or doesn't love you...