Scientists now have reason to believe that kissing was created to spread germs which build up immunity to illness.
The loved-up gesture allows a bug, which is dangerous in pregnancy, to be passed from man to woman to give her time to build up protection against it, reports The Telegraph.
The bug, Cytomegalovirus, is found in saliva and generally causes no problems. However, it can turn nasty if caught while pregnant and can kill unborn babies or cause birth defects.
Researcher Dr Colin Hendrie from the University of Leeds, said: "Female inoculation with a specific male's cytomegalovirus is most efficiently achieved through mouth-to-mouth contact and saliva exchange, particularly where the flow of saliva is from the male to the typically shorter female."
He wrote in the journal Medical Hypotheses that kissing the same person for about six months provides the best protection. And as the relationship reach new levels, the kisses become more passionate, which results in building up of the woman's immunity and cutting her odds of becoming ill.
By the time she becomes pregnant, the odds of her unborn baby becoming infected are much lower.
Dr Hendrie said: "Information concerning body tone, smell, reproductive condition, disease state and, of course, personal physical and oral hygiene can all be gained solely from close physical proximity.
"The small amount of additional information from kissing is an unlikely pressure for its development."