Seth Roberts speculates:

We get a small amount of pleasure from touching our mouths. The pleasure declines after it is “harvested” and takes several minutes to become available again. This mechanism evolved because it kept our lips moist. At the time it evolved, people spent little time sitting. The pleasure was obtained by pursing or licking your lips, which moistened them. Predictions: 1. if you watch people whose hands are busy, they will purse or lick their lips roughly as often as people in an audience touch their mouths. 2. The more you lick or purse your lips, the less you will touch them with your hands. 3. The more you touch your lips with your hands, the less you will purse or lick them.

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