I might never fall in love with someone's smile because there is no way for you to know what a splendid smile could hide.
However, I could easily fall deeply in love with the expression of someone who's lost in his own mind. The faces he makes when he thinks that no one is watching are unique.
You would see his truest smile while he's remembering a funny situation or witness the purest of his tears as a sad thought crosses his mind. He could also just be completely and utterly lost in a world of his own with his eyebrows furrowed.
He would surely be unable to reproduce any of these pure expressions although these are what make him who he truly is deep down.
In that exact moment, you would be able to read his soul, and this, my dear, is what could make me fall.
I fall in love with souls.