Cheers...

This may be a Standard Life lift phenomena but....

Picture the situation. You get in a lift. Someone else you don't know gets in at the same time. You're going to the 4th floor, they push the button for the 3rd. The lift gets to the 3rd floor, no words spoken. They depart the lift and say "Cheers" or "Ta".

This happens all the time. Why? Thanks for accompanying me. Thanks for allowing me to leave the lift.

I don't get it. It's like the drivers that stop on the single lane road to let you pass first and then wave a thanks as you pass. I should be thanking you (I am a courteous driver and generally do.), not the other way around...