Is Thank You so hard to say?

Thank you. Is that so hard to say? I am more than willing to do favors for or help people but it gets annoying when certain people never say thank you. I was raised in an era when people were polite to one another. It was especially well taught to me by my parents. Having been raised in the deep south it was just natural. I thought I raised my children in the same way, but I guess I failed with one of them. I have a daughter who occasionally asks me to keep her children for various reasons. I have yet to get one small "thank you, mom" from her. I just dont understand.