It is because of the evolution of primates. If you read Desmond Moris's The Naked Ape, you will understand the reason. For those who don't know the reason, we don't have fur, feathers, or anything else that can be used to attract a sexual partner, so our lips act as a signature, a call sign to announce what's down there between the legs. If you have larger lips, you have better chances of recognition among your peers. People who live near the equator have dark skin, so it is hard to distinguish the lips. That's why Africans have prominent facial features; they are all the instruments of finding a better partner for sex. White people have lighter skin, so they have a very bright sexual call sign since it is easy to show your facial features. When you are aroused, the lips and cheeks change color with the blood flow. Still, men like it if their partners have large breasts, big buttocks, and large hips, and women like tall guys.