I think the answer is very very simple but at the same time very very complicated. I guess that it's not like hand in glove but a multi-dimensional instantanious thing that you know is right but can't cross reference all of the reasons in the same instant so it seems weird. I suppose that if we could express what the 'things' are that would make a relationship work then dating agencies would be out of business. Of course physical attraction is in the mix but taste like food, music, literature and art all help define the person that matches what you are looking for / need.
I feel very blessed that of all the people rolling around in this great big world I managed more by luck than judgement to meet the right one, the very one. You could argue that who you are and what you do puts you in a position more often to meet the right person but that still has a huge random factor built in that must mean there are so many near misses it's frightening.
In short we didn't 'know' we were right for each other not completely not for sure but we felt we were right for each other and were lucky enough to be proven right. Great question!