Why do people think were married?

A female friend and i have known each other for about 10 years now. were nothing more than really close friends (maybe benefits every once in awhile) but were not dating in any shape form or fashion. we are constantly joking, poking and prodding at each other for fun 24/7 in the store, grocery shopping or just anywhere it doesn't matter but most of all we LOVE arguing just to get under the other persons skin. people think we are married for some reason. why is this?