I feel its to do with the other half of your lips constantly being moist from being in contact with the inside of your mouth.
I have tried various times to dry out my mouth by keeping it open and consistently sucking out all of the saliva from my mouth. In doing this, the whole of my lip appears to heal in one go. The drying out of my mouth/lips is helped by adding a compeed coldsore patch to the triangle of my top lip (a recommended treatment I read on this forum).
Due to always having to speak to people and not having a lot of free time, I have never been able to do this treatment for more than a day, and the lips always revert back to normal. But I am looking forward to trying it for at least 4-5 days minimum.
I recommend others to try this treatment as I have seen temporary results which I believe could be effective if continued for a longer time.