Chemistry - the science of love?

Why do they call it chemistry? Is it cause of all the chemical reactions that take place in your body when you meet someone? Or is it the chemical reactions between you? Pheromones in the olfactory bulb on your cribriform? Or the endorphines that run through your veins? Reflexes that make you vasodilate, blush and feel butterflies in your stomach? 

Or, did some random guy, a long time ago, think of this as a reason to run/go into a relationship? It's something you can't really argue right? If he says,"we can't be together cause we lack chemistry (aka: the spark)" or when he says, "I think we should be together, cause we have so much chemistry". 

It's not quantitative, sort of qualitative and it's absolutely subjective. 

Question is.. Is chemistry enough? Can it last? 

I reckon every start of a relationship needs a little bit of chemistry.. Nothing would really work when the science of it doesn't work right? Simple equation, addition of reactants  - chemistry - reaction. I honestly wish it were that simple. 

My head. My heart. My head. My heart. My head. My heart. 

One's trying to out rule the other and I don't know which is going to win really, but I reckon only time can tell and will tell if my heart and my head can come to common consensus, which I think, would not be too hard a thing to do. I hope God will speak to my heart and lead my head to His will!