The girl, who was wonderful, asked why I said please when I asked her to do something like empty the dishwasher. And it would be followed by a thank you when she did.
I could have answered that my grandmother would rise from the grave if I weren't polite. I once was rude to a waitress in a restaurant. I could not have been more than nine. My grandmother yanked me out of my seat and put me in the car. She could see me out the window while she finished her meal. "Never be rude to someone who is not able to answer back," she said.
There's another reason: please and thank you show respect.
I will say thank you to a waiter when he brings my food. I will thank someone for responding to me in English when I speak French, although I sometimes ask if they think I have an English accent. I love the incredulous looks I get. "Un petit peu" is usually the response.
Although Rick brings me tea in bed most mornings, I wouldn't dream of not thanking him. Yes, it is a routine, but such a lovely one and he expended effort for which he should be thanked.
We both thank each other for meals the other prepares or snacks. The same with anything that shows consideration.
Sometimes when dealing with internationals I will say merci, danke, gracias or shukren or all if I'm not sure of their nationality. This usually brings smiles. Once, it led to the most incredible discussion on a flight from Prague to Geneva, a true moment of sharing between two women from very different places but identical values.
My grandmother had another theory about words like please, thank you and excuse me besides showing respect for another human. She believed politeness greased the wheels for social relationships no matter if it was a superior, a stranger, a service person and/or most importantly a family member.
My grandmother when she was young.
Throughout her life, she was a stickler for manners.