Is there anything more frustrating than when you're having an argument with someone about, say, parenting in medicine, and they other person says to you that "you have no idea what it's like to be a parent"?
I mean, how do you respond to that? Not only is it really condescending, but it shuts down the conversation entirely.
Now that I'm on the other side of things, I really try not to ever say that because I remember how obnoxious it was. But the truth is, it's difficult. Because sometimes you're talking to someone and you realize that they really have no clue what they're talking about. And it's frustrating. So it slips out.