Why do people get angry when...?

Why do people get angry when I act like I am of a culture that isn't mine? I'm adopted and I was told I was Irish my whole life by my adoptive parents(who are Finnish) and I found my birth parents only to find out I am really German. So if I act like I am part of any of these cultures why do people get angry as if I stole something? I really don't understand this because I've had to act like I am from cultures that aren't really mine my whole life just to keep people from getting their panties tied in knots and allowing the mindless populace to think the world is simple with clearly defined catagories. Can anybody tell me why it's a horrible thing to act Irish, Finnish, when your really not, because I've had to do it my whole life and I don't know any different. Please help.