Here is a scientific answer I found...
now for the science bit » Why do we close our eyes when kissing?
I used to open my eyes... then I realised almost everyone I know closes theirs...?
Scientists have been trying to figure this out. Yau Che Ming, a professor at the National Education Institute in Singapore, has a few theories: one is that closing our eyes eases the strong emotional pressure that we feel while kissing. This clashes with others who think that we close our eyes to focus on these emotions. Professor Ming also hypothesises that being so close to our object of affections face alters our visual perception of them, either making it too blurry, or too clear, and so we close our eyes to avoid this. But this mathematics, technology and physics teacher hasnt published his research, nor performed experiments of people kissing (that we know of), so not many useful conclusions can be drawn.
Sheril Kirshenbaum, a scientist who wrote an entire book on the topic,
The Science of Kissing, explains that during novel experiences, and pleasurable experiences, our pupils dilate. Even just looking at someone were attracted to makes them dilate (I would love to test this maybe by putting cameras on the heads of speed-daters although that may kill the vibe).
Im not too sure on this one. Pupil control is much more complicated than that. Yes, wider pupils let in more light, and may cause us to close our eyes, but how is this evolutionarily beneficial if we close our eyes when spotting a potential love interest?
My theory is that if your kissing partner were to open their eyes while yours were open, one of your eyes would take up their entire visual space, and this would be as freaky as swimming alongside a blue whales 15 cm diameter eye.
Kissing is probably the most personal thing you can do, more personal than sex (prostitutes dont kiss because its so personal well, at least thats what Ive picked up in movies), and so our reasons for closing our eyes while delving into someones mouth are also likely to be very personal.
So why do we even kiss at all?
The debate is about where kissing comes from: is it learned or instinctive? The learned camp say that in the days of our early human ancestors, mothers chewed food and passed it from their mouths to their babies, and continued this action of pressing their lips to their children, even as they grew up, as a sign of comfort. Also, 10% of human tribes around the world dont kiss, suggesting that its not instinctive, but is learned. Indigenous people in Australia, Malaysia, Tahiti and some areas of Africa began kissing only after Europeans settled in their areas. In China, kissing is replaced by rubbing your nose on your lovers cheeks.
There are those who think it is instinctive, since many animals seem to kiss. It may just be a form of grooming, but some animals, like bonobos, swap a lot of spit.
Chimps kiss too. And swapping a mouthful of germs could be beneficial to our immune systems, like a vaccination boosting protective responses.
The most widely accepted theory of kissing is that we do it to suss out our mate. The sensations of touch, taste and smell are involved, and studies have shown that women subconsciously like the smell (pheromones) of men whose immune system genes are different from their own, yielding children with stronger immune systems (1).
Studies in monkeys have found genetic qualities in their odours, which can be used by potential mates to suss out their compatibility.