The surface of our lips is different from the surface of the skin on the rest of our bodies. Our lips are made of the same mucus membrane that coats the inside of our mouths. This means two things: First, our lips don't have the same protective outer layer as the skin elsewhere on our bodies. This layer is part of the barrier to evaporation, and its absence means our lip skin retains less fluid. Second, our lips don't have the same glands and pigments as ordinary skin. For example, lips don't have oil glands, which also help keep moisture from evaporating, and they contain less melanin, the pigment that helps protect our skin from sunburn.
Don't Get Chapped
What this means is that our lips have little to no protection from the environment, and their main source of moisture is the saliva that comes from our mouths. What's more, unlike much of the rest of our body, our lips are almost always on display, and rarely covered by clothing. As a result, the moisture on our lips is continually evaporating. And that's before we take into account exposure to sun, wind, cold and air-conditioning. Plus, if you're sick or have allergies, and your nose is congested, your lips end up drying more quickly because you tend to breathe through your mouth.
With the odds stacked against your lips, it's a good idea to keep lip balm on hand and keep your lips moisturized.