[@Mistress Dizzy] They don't necessarily have to wind up together. There's really no reason not too, because for the most part everyone is extremely compatible with their Destined - the gods made it that way. However, people can fall out of love, but that is just as bad as not finding your Destined, because you will lose your powers when you do. So yes, stuff can go wrong, though mainly from external factors. 

On the other hand, people can totally fall in love with someone who isn't their Destined. It's pretty common because it's so hard to find your Destined. This however does mean that you lose your powers, too.