Many people believe there is a certain someone out there, a person "just for them." Like two celestial objects, the perfect pair drift through life until circumstances contrive to unite them. I think it sounds nice, and after you have found your person, the process makes a lot more sense.
Personally, I have no problem in people reinforcing their relationships with the notion that they were fated to be together. It seems a little quaint, and I think fatalism is a questionable quality whatever the outcome, but as far as emotional crutches go, this one is pretty harmless.
By the light of reason this idea seems rather silly. There are now more than six billion people on the Earth. How fate works to designate whom should be paired with whom remains, for reasons I'm sure are readily apparent, somewhat unclear. If the distribution is purely random then you end up with some rather bizarre results. Not only is it unlikely that you'll ever even see your predestined partner, but language, culture, age, and other concerns can make the prospect of such a meeting uncertain to say the least.
Practicality may force the concession that our amorous statistics naturally engineer it so that our fated lovers are geographically and chronologically compatible, but lacking any evidence of such manipulation it is an offering as strained as any to be found in pseudo-science.
I'm not really sure what the point of all this is though. I certainly don't begrudge people their innocent delusions, though I think the thought process their support entails is questionable. I guess ultimately I don't feel love is any better a subject to leave to fate than anything else.