There is something about the dark that makes you reach out and explore. Life itself is a composition of dark times and happy days but the best days are moments after the dark times. The moments you feel the rush of relieve and start to see things in a new light. Darkness is associated with not seeing and seeing is to most people, the most important sense. However if the closest thing to seeing is feeling, then we can say that to feel better you have to shut down the sense of sight.
Why then would we wonder why people close their eyes when kissing? The many distractions to the eyes are enough to prevent you from feeling the full impact of a kiss. You already have distractions in your mind that you have to block out and let go; adding that of sight is not advisable. As I once heard if you are kissing and your partner leaves his or her eyes open then He or She is sure not that into you.
Shutting out the light in an alley would force you to feel with your hand in order to move forward. A good kisser explore with the tongue, finding ways to caress every nook and cranny of the mouth. How much you can explore is what makes you a great kisser but it all start with shutting your eyes.