Well I watched a video of some ukrainian matchmaker at youtube and she shared her experience she made with her clients. Many men and women went on dates and all went well, it was first and sometimes second date as well, men liked how the women looked like, she was beautiful, they enjoyed conversation with each other which was also perfect, they talked for hours, they found many things in common, they liked the same films, did the same sport, etc. but after that men didn't want to continue and the matchmaker wondered what went wrong when everything was so perfect and the reply she got was that there was no sparkle and no feeling she was the one. And then someone commented talking about chemistry like this:
"Chemistry is when you get that feeling when someone walks into a room and you get that electric feeling, alertness course through your body which you don't get from most other people. Then when up close that intensity of eye contact usually quick eye movements. This way you know she feels the same way but of course there is usually awkwardness in being in each others presence because of it. You can of course have a relationship on other grounds work out, on personality compatibility, a strong liking for each other, pleasant companionship but I think long term these may have the potential to be mistakes."