"Predestined Affinity" would work if you were talking to a chemist or scientist. 'affinity' (chemistry term) means --> "drawn towards" and used in entropy equations. Some people would not find it romantic. Most people would just say this in simple terms : "meant to be together" you could also play with these phrases: "familiar attraction" "drawn to you from a deep sense of knowing you" "past lives together" "we have some kind of karma (from our past)" have fun!

Simple. You don't.

缘分 = fate or chance that brings people together / predestined affinity or relationship / (Budd.) destiny This question is asked a lot here. I recommend using "predestined affinity". Those two have a predestined affinity. Or there is another saying: Our love/friendship was made in heaven.

In 2 words : Predestined affinity

Fate差不多吧。 比如... It was fate that brought them together. 他们俩有缘分。 这边有更多的例子： http://zhidao.baidu.com/question/52016118