You and your soulmate are connected by a red string that is tied around your pinky fingers.
The catch comes in as everyone has one, and they get tangled very easily.
Every once in a while, fate likes to give you hints by pulling on the string.
As you become closer, your string becomes less and less tangled with others until you realise they are your soulmate and your strings lead straight to each other.
For a fun twist, if a soulmate cuts the string, it acts similar to the string of fate in Greek mythology, and the cutting of a string symbolizes a person's death

(This one has many versions, but this is my take on it, except for the end, that was just my idea as I was writing this, because I'm an absolute nerd for angst, and mythology! Well, honestly, only the first part is the widely known thing, the rest of it I made up on the spot, but I do that with a lot of these, so what's new?)