Why do we touch our mouths so much?

Seth Roberts asks:

The photo shows the full faces of 22 men; 7 of them are touching
their mouths. I have noticed something similar at many faculty
meetings. I started to notice this after I read about its observation
in a study designed to measure something else.

I’ve known about this for many years but have never read an
explanation. Do we enjoy touching our mouths – or is the absence of
touch for a  long time unpleasant? If so, why?

Here is one poker player’s answer.