This mysterious force that bonds two people is called 'Chemistry' because it is difficult, like a chemistry class.
Even while I actually am experiencing 'chemistry' with a potential mate, I can not figure out why and how I have chemistry with that man and not the other men.
It just happens and I can not explain it logically in physical senses.
However, I think that fate/destiny has something to do with it.
Chemistry exists between two people so their destiny can be fulfilled.
It doesn't mean that the relationship will last their lifetime nor that the relationship will be a happy/healthy union.
But they are meant to be together for whatever pre-determined time period and for whatever purpose.