How to react when someone calls you sir?

I have observed the word sir is very prevalent in indian society. I always used this during my school and college days for the teachers but as I got into corporate. I found many senior people some even 40-50 years of age but when I told them as sir, they asked me not to do so as this is not culture here in the company. I understood and started calling everyone by first name and I also felt very comfortable about that. Now cut to the moment when I am myself at receiving end, wherever I go to hotels, any clinic, resturants or anywhere specially in cities, I am greeted with 'sir' even by many people who are very much older than me. And funny thing is I look very young for my age, like college going teen. So I get so embarassed and cringe from inside that how and why are they telling me sir. How do you feel about this sir culture in indian society?