Just like saying please. Just because you say please doesn't mean you get what you want. And yet we start early to teach kids to say please and thank you and they associate it with getting what they want. You want a drink, say please, You want the book say please. Child says cookie please and you say no. Child gets confused and stops saying please and then we get upset with them. I know we want to raise nice polite children but we have to be very careful about how we go about it and stop putting adult words and behaviours onto them. I still say please and thank you when it is appropriate but don't expect them to do it.