Nice and polite parents teach their children to be the same- to be nice and polite to their family, their relatives, their teachers, their friends in school, to the elders, and to strangers. It will just bring out the best in them and it makes their parents proud.
My daughters are taught the same way, it will develop them to better human beings.
My younger daughter's boyfriend (they are both 16) is very polite and shows good manners when he comes over for a visit.
He does the following to me and my home:
1) acknowledge that he has arrived with a warm hello and a hug.
2) offers to help out in the kitchen.
3) says a silent grace before dinner.
4) does tidying up of the sofa where he had used it overnight.
5) says "goodbye" and "thank you" when he leaves.
I see that he has a good up-bringing and this reflects to the parents.