What is it about a large pane of glass that makes people want to touch it?

i work part-time in a sandwich shop, and we have a glass divider between us and the customers. people are constantly putting their hands all over it. i cannot understand why they feel the need to touch the glass. there is no reason for it and it just makes extra work for me and the girl i work with. why can’t these grown folks keep their hands to themselves? do you touch the glass? if so–why? also, when opening a glass door, do you use the handle or do you put your hand in the middle of the glass and push? and do you realise that someone has to come along behind you and wipe off your dirty handprints?