Over a lifetime one creases one's lips with certain expression more than a million times and eventually with time the lines just stay there.
The vermilion border of the lip becomes distorted and "bleed" lines develop. In fact, any activity that makes your lips pucker, which activates the orbicularis oris, a purse string- like muscle which encircles the mouth, can cause wrinkles to appear on the upper lip. The development of such wrinkles is a function of both the activity and how often it is done. Smoking, talking, whistling, drinking water from a bottle and chewing gum amongst others, all cause the mouth's main muscle to contract. With time vertical wrinkles develop and the severity of these wrinkles will depend on the thickness of your skin and the size of your lips. The thicker your skin, the less likely you will develop them. This is evident in men who rarely develop lip wrinkles as compared to women who may develop them despite being a non-smoker.