Where did it come from? Why did we do it? When was it first documented? Did we do it everywhere across the whole world or does it stem from one region?
I would really love to know this as well. All I know about it is that in french its called "un baiser amoureux" - A Lovers Kiss or "un baiser avec la langue" - A kiss With The Tongue. I did hear a rumour that it originated from Florence in Italy and the french used to call it "baiser florentin"
AskHistorians "probed" this half a year ago [in a thread about tongue kissing among the Greeks and Romans] (http://www.reddit.com/r/AskHistorians/comments/1shvji/regarding_sex_ancient_greecerome_and_the_golden/). This should give you a good start.