Rudeness to me is mostly less about what you do and more about how you do it, as for why...lots of people are walking around with trauma and shit, you walk up to someone you don't know and start commenting on them (or it could be someone you do know, just not well enough), you never know what nerve you're going to hit and you should be ready for that and maybe a little understanding instead of being appalled that someone feels a certain kind of way about what you said, whatever your intent