What is required in many different situations in American culture? An expression of______.

(16) Many different situations in American culture require an expression of thanks. We thank people for big favors, (17) such as picking us up as for small things, such as holding a door open for us when we have our arms full of things. Thus we say "thank you" all day long. In general, we thank people for giving us a gift, doing us a favor, offering to help us and so on. Although we thank people for many things, (18) excessive expressions of thanks are not necessary. In fact, they make Americans uncomfortable. For instance, if your adviser spends an hour of his time helping you edit a paper you have just written, you will want to say, "Thank you for your help. (19) I really appreciate your time." However, that’s enough. If you keep on mentioning his kindness, (20) he might feel uncomfortable and annoyed, and possibly will not be eager to help you again.
Q16: What is required in many different situations in American culture?
Q17: For what do people say "thank you"?
Q18: What is not necessary for other’s help?
Q19: What will you say when your adviser helps you edit your paper?
Q20: How might your adviser feel if you keep on mentioning his kindness?