Why do they do it? read all?

Maybe you do it unconsciously so while others notice it you don't. Try to be more aware of how you look at people because it's not just one or two people that knew you who said it, complete strangers said it too. It must mean you have a snotty look somehow without knowing it. Try smiling more and when people say that you're giving them the look tell them you don't mean to do it and don't mean to offend them. Poor you :P smiling more should help though, hang in there ^-^