Genetics:  Your lip shape and size are inherited.  (Thanks Mom!) If you have thin lips to start though, thinning will become noticeable at an earlier age.
Lifestyle:  Both smoking and drinking from a straw can create wrinkles and thin lips.  We already know why smoking is bad for you, but straws are recently banned in California, so it is a great time to give up that habit.  It hastens the thinning of your lips.
Age: The same aging process that occurs in your skin happens with your lips.  As you age your lips begin to lose collagen and elastin. You also have less hyaluronic acid which keeps lips plump.  Over decades, we lose bone density in our face and have wear and tear on our teeth which can lead to a loss of support of our lips.  This can make them appear sunken in or less full.