chemistry...what is it

So what is it that creates "chemistry" between two people.
When does it begin? How does it build? The chemistry felt....is it the same feeling across board with anybody met?
Or in some instances a spark different ignited ....by....what?

Words....it is true....mightier than the sword....it can bring a nation down
Or
Through positive reinforcement build an empire strong.

And thus, the initial " hello"....? the meeting of eyes....? an intangible feeling...
Could it be as superficial as looks...a nipple revealed, a compliment paid?  Are some people more susceptible and open to the "pheromones" of chemistry....a state of mind inviting....searching a kindred spirit to share in feelings well below the dermis.
Are the eyes the receivers, the ears...?
Time?
Is it the long periods of drought where nothing of value has fed the soul, where a sad yearning of being "seen" and simply acknowledged, all of a sudden ignites an awakening.
A  soul knowing in the giving of kindness....non-threatening in their demeanor, rather than the taking, inspires one to open up and share of oneself without hesitation.
Bang!
Chemistry?
And then .....does chemistry last and is it a stable and sound foundation to build anything concrete on?

RB.