why is a kiss marked 'x'?

Its actually a Norse rune galled 'Gifu' which is basically a good luck charm. Bakers used to mark special cakes with it, you see it on old door frames sometimes, and we use it on letters as shorthand for 'I wish you luck' which has changed through time to be a shorthand for 'kisses' if you put three 'kisses' you are invoking a charm of three, effectively writing a simple good luch charm to bring triple luck and love to the person you send it to!

I believe it came from the days when a lot of people were illiterate and so signed their name with an "X" to all documents.

never heard that before...... i've just heard X marks the spot!.. now whats up with kissing ...... have u gotta mark an X before u kiss.! LOL