Because your lips are comparatively thin, they're also more exposed to damage from seemingly innocuous activities. Case in point: licking your lips.
It makes sense that licking your lips should moisturize them, and it does -- for a moment. As the saliva your tongue delivers to your lips dries, it evaporates, carrying with it the natural moisture found within your lips. What's more, your saliva is composed partly of enzymes that are designed to initiate the process of breaking food down for digestion [source: Gardner]. What works on food also works on the skin of your lips; two digestive enzymes found in saliva, amylase and maltase, can actually degrade the skin. This leaves the already thin skin of the lips further exposed, creating even drier lips.