lips are one of the female features emphasized with makeup and even injections of silicone or one's own fat from other places on the body. Women know that men unconsciously recognize full lips as a sexual signal. The more sexual hormones a woman has, the more full her lips are.
Lips in both men and women swell with additional blood when they become aroused. (as well as other body parts!) As men and women age, their lips become thinner because their sexual hormones decrease in later years.