Or could it be a evolutionary form of stress testing your weaknesses? A bit like the urge to suck on a cut you have on your arm. Get that saliva in there to heal the wound.
As your probably know, saliva acts as an anaesthetic. No TCP back then! In fact, my great, great, great.....great aunty lived in a tree. Basic living conditions! In fact, social security back then was a mere olive branch. [Hangs head in shame.]