The brain functions in a very funny manner. Even though touch, sight, smell etc may all be different senses,using different nerves and brain regions, focus on any one sense might get sharper if the other senses are dampened.
Kissing is a tactile sense and seeing a visual sense. So closing your eyes to “cut out ” the visual sense might focus all the energy on the task at hand.
However, it might also be a learned behavior, where people close their eyes because they have seen in movies etc that that is what happens. I wonder what happens if a person not exposed to TV etc before gets in such a situation. Or your first kiss?
I wonder if it is true, or a myth that blind people have extraordinary other senses-compensation?