"Because I said so and I AM YOUR MOTHER!"
This brings to mind many an argument with my own mother starting at a very early age. Somewhere in a box in my attic is a journal from my childhood where I actually documented some of the phrases I swore I would never say to my own children when I had them. You may ask why I haven't tried to find it. The answer is that I am scared to see what I wrote. I've only been a mom for two and a half years and I'm guessing I've said at least 75% of things I swore I never would. My mom said she would place a large wager that not only some of these comments, but all of them, would come out of my mouth when I too had children.
These stupid phrases must be ingrained in our heads at birth, and therefore we do not have the ability to control their being said. This is the only explanation I can come up with because I hate to give my mother the satisfaction of being right.