—>interesting points from this link

• In a good kiss, our pupils dilate, which is one of the reasons we close our eyes,
• Women are actually most attracted to the natural scents of men who have a different set of genes called the major histocompatability complex that codes for immunity. We’re most attracted to people whose MHC genes have a lot of diversity from ours—the advantage of that would be if you reproduce, that child’s probably going to have a stronger immune system, and so be more likely to survive to pass on their genes. This isn’t something that we’re consciously aware of, but we do seem to know if something feels off.

• If it’s a good kiss, there’s an associated rise in the neurotransmitter dopamine, which is associated with strong feelings of craving and desire. Kissing also stimulates the same pleasure centers in the brain that are associated with addictive drugs like cocaine, so kissing really does act on the brain and body like a drug. There’s also a rise in the hormone oxytocin, which is known as the love hormone and is important in terms of social bonding and really committing to someone, and serotonin, the neurotransmitter that is often associated with obsessive-compulsive thoughts. It was really interesting to me as I was writing this book and learning about the chemicals coursing through our brains and bodies, to discover that when we think about these symptoms we associate with being in love, well, there’s a real chemical basis for that.

• Women can actually take in testosterone from a man’s saliva during a kiss. That was so funny, because I kept coming across people saying that too much tongue is gross, and that it’s kind of silly, but there’s actually an evolutionary reason for it. It’s to a man’s advantage to be slipping a woman the tongue. He’s of course not aware of it, but he’s giving her a bit of the male hormone testosterone, which over the weeks and months might enhance her libido, and she might then be more receptive to sex, so it’s to his benefit to do so.

• If the timing is bad, or if you’re feeling pressured, you’re going to get a stress hormone called cortisol in your body, which really isn’t conducive to a good kiss.