I'm like that. Even when I don't feel comfortable using the familiar with someone, if they consistently address me that way I'll start finding my use of the formal somewhat awkward. It might have something to do with the fact that my primary linguistic socialization was in English, but even there you have the first name thing going which I find annoying from people who I don't know and am not interacting with in an informal setting, but I still tend to swallow my discomfort and go along.
As the older rock critic reminded the young protagonist in Almost Famous ... they are the enemy (or maybe that was a member of the band telling the protagonist 'you are the enemy'). I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.
