Do you drink coffee? Cola? Wine? I'm a big coffee drinker, and I prefer wine over other alcohol, but I've given up cola. Why? It stains my teeth.
Cola and I parted company years ago, when I realized that my coffee drinking and my cola drinking were having a negative affect on my pearly whites. It might be that I notice these things more often as I get older, or it might be that I notice these things more often because in a business setting (meetings, conferences, networking events) the first thing you notice is someone's smile.
If the eyes are the windows to our soul, the smile is the sun shining through. When your smile is dinghy, it reflects poorly. When it shines, you send a positive, "hey, glad to see you!" message that is not overshadowed by... a shadow.