I think it's definitely chemistry. I even think body chemistry/smell/genetics may play a big role. And I think both parties have to feel 'compatible' and not just in bed.
But why we have it with some and not others? I've thought about that a lot. Pre-bang, we talk about having a 'crush' on someone and that goes both ways, we've all been there.
In europe, they talk about being 'cunt-struck' with a girl post-bang, instead of love-struck since you're not in love. But you feel this certain...je ne sais quoi....with that girl. You enjoy being around her even when you're not banging, even though you want to bang her noon and night and so does she.
I was nailing this sexy, sorority athlete for around 6-7 months some years ago and I still think about her at times, and I'm the one that ended things. It started as a massive crush on both sides, then moved into dating. We definitely had a major bond, we stayed in touch for a few years after she'd moved away, even hung out a couple times in different cities although we were both in new relationships. Usually I can't stand to be with a woman after I've dumped her and have no interest if she's dumped me [unless it's straight come over & bang, +she's really hot].
Why was this one different? I have no explanation. She wasn't the hottest nor the prettiest nor the tallest nor the kinkiest nor the most fashionable nor the most feminine -- she had great sex appeal and talent, but I've had other girls like that I forgot about real fast. Lot of girls give good head.
So...maybe it's just 'chemistry,' after all. If I knew the answer I'd write a book.