Your lips are full of muscle, and the skin on your lips is the thinnest part of your body and more delicate than any area. The layers of skin on our lips are only 3-5 layers thick - which is why our blood vessels are more visible in that area and give us that nice pink or red lip colour.
Moreover, our lips do not have any oil glands as they don’t contain any sweat or sebaceous glands. This is why our skin is not protected and seem to get dehydrated or chapped through external aggressions so easily.
Another aspect to look at when thinking of our lip's skin is that, when we are born, that area is full of collagen, but as we age, we lose the plumpness of this collagen (it can happen as early as 16-years-old!). Our bodies will produce less protein in the lips as we age, and show signs of thinning and less volume. In short, looking after your lip area is not only about avoiding dry lips: hydration is also the key to healthy, volume lips!
#funfact Another interesting fact about our lips is that they are unique - like our DNA, our lips are not identical to anyone else's!