Do you believe there is such a thing as sexual chemistry?

I think we all know of cases where two people seem to have a lot in common, are good friends, feel great affection for each other yet do not have any wish to be anything more than that. Sometimes one person wants more and the other doesn’t. Assuming both of them are unattached and looking for a serious relationship, would you explain this situation in terms of a lack of sexual chemistry?
How do you experience and explain attraction or lack thereof in your own life?

Have you ever wished you could be attracted to someone because rationally they would be so right for you, but there was just no “chemistry”?

If you do believe in sexual chemistry, what would you say it is?