I was a military brat, so "sir" used to make me think "You must have me confused with my dad." As I get older, it doesn't bother me, and in some cases I require it. When I used to teach, I told the kids to address me as 'sir' or use the Mr. It seems, however, to be a trend to address teachers by using the Mr., Mrs., or Ms. followed by their first name, and one kid asked me on the first day of class what my first name was, to which I replied, "Mister. My first name is mister." For the rest of the semester, I was Mr. Mister.