Although not proven, these theories suggest:
1. Lips being sensitive so possibly due to anticipation for procreation
2. It’s a learned behaviour carried on through the ages when mothers chewed food and “baby birded” into the toothless babies their food which continued with pressing against their lips/cheeks when they were older for comfort
3. Due to evolution, being a social organism, kissing may have resulted from they
Although not proven, these theories suggest:
1. Lips being sensitive so possibly due to anticipation for procreation
2. It’s a learned behaviour carried on through the ages when mothers chewed food and “baby birded” into the toothless babies their food which continued with pressing against their lips/cheeks when they were older for comfort
3. Due to evolution, being a social organism, kissing may have resulted from they
More info here: https://www.livescience.com/32464-why-do-people-kiss.html & Why Do We Kiss?YouTube · Vsauce · 1 May 201312:16