When you buy something you might encounter an awkward exchange: you say "thank you" to the cashier and the cashier says "thank you" back. While strange at first, its evidence that trade is mutually beneficial. I got something I want (say, carrots), and the store got something it wanted (money). So we thank each other.\
A student just dropped off her homework assignment. I thanked her and she thanked me back. Does this mean that homework is mutually beneficial as well? That seems strange. Homework for me is more stuff to grade. How do I benefit? How does she benefit?
Why am I thanking her?
1. Perhaps it's a habitual reaction for when anyone hands me anything. But I doubt that if someone handed me a meaningless set of forms to fill out, I'd thank them.
2. I have a professional obligation to grade this homework. By turning it in, she is enabling me to fulfill my obligations. But that obligation only occurs if she turns it in.
3. By turning it on time, she is making my life easier. But I would probably thank people if they turned it in late.
Why is she thanking me?
1. She is appreciative in advance that I will grade it, but I have a professional obligation to grade it. Still, maybe she is appreciative that I am being professional.
2. She is appreciative that I have accepted it to grade, but it was on time so of course I will accept it.
3. She is thankful that she now longer has to work on it, perhaps thanking me that it wasn't longer/harder than it was. This might be true--I get less thank yous for harder assignments--but it doesn't pass the smell test.
A fourth possibility exists for each: mutual thank yous is an acknowledge of a passing of responsibility. She is pointing out she is turning in her assignment and that she has finished it to her satisfaction. I am acknowledging that I am accepting her assignment and it is my responsibility now to grade it.
If this fourth possibility is true, then it calls into question why we have mutual thank yous at stores. Perhaps it is a social convention left over from before receipts: the owner is acknowledging I can leave the store with the item and I am acknowledging that he can take my money. David Graeber points out that the first form of money was debt (as in a series of favors). "Thank you" is an acknowledgement that a favor is now owed.
I find (1) from each side to be most probable but (4) to be most interesting.