In this context there's absolutely no supernatural component whatsover, this is just about human beings. And I think a soulmate can definitely be temporary, in a sense... human beings grow and change constantly, who you have a deep connection with at one point will be as distant as a stranger later on. Makes complete sense. (same in even romantic relationships, which is why a long-term thing is hard work... making sure you keep growing in the same direction so that you never become strangers to each other. just ASSUMING you'll always have that exact connection just can't work)