I recently re-read one of my favorite series (the Mercy Thompson series by Patricia Briggs), and there are fae in the books. One of the things that is mentioned quite a few times is the prohibition against saying thank you to the fae. Gratitude could be expressed, but you couldn't actually thank the fae (or they would be able to use your thanks as a means of controlling you).
This is a pretty common belief about the fae, and it got me thinking about why it would be bad to thank them. Another common belief about the fae is they can't lie. Put them together, and I think you have an interesting reason why a verbal thank you would be undesirable, while showing gratitude was okay.
Think about how many times in a day you say little things (like thank you, I'm sorry, it's okay...) without really meaning them. I do it all the time. Sometimes I do feel like I may have inconvenienced someone else, like when I say I'm sorry as I move my shopping cart from where I left it blocking an isle. Sometimes it's a knee jerk reaction in social situations, like when I am standing still and someone else bumps into me, and 'excuse me' has already left my mouth without my even thinking about it.
I'm not always truthful in these little bits of politeness. Sometimes, when I say that it's okay in public, I am thinking quite nasty thoughts because it really was kind of rude whatever the other person did....but I am not going to make a fuss because it's just not worth it to get all worked up about it, so I apologize or accept an apology, rant in my mind, then move on.
But back to gratitude and thanks. I think we tend to say the words thank you without much meaning behind them. And worse, I think we sometimes say them as if just saying the words is enough to show how grateful we are. And sure, sometimes just words are enough. If we drop a pen at the bank, and a stranger picks it up and hands it to us, a thank you is probably sufficient. If our neighbor comes over to our house everyday for a week to take care of our pets while we are out of town on vacation, does a mere thank you really cover it?
Words are important, they really are. Think about a day where you were just having a really rough day, and everything was going badly. If you happen to hold the door open so the lady with three fussy kids can get through, even though her hands are full, and she looks at you and says thanks....it makes you feel better, even if it is just for that moment.
But actions are important too. Sometimes even more important than words. If we can show how grateful we are, it leaves a bigger impression. Instead of just saying thanks for someone who has gone out of their way or inconvenienced themselves to do something for you, find a way to show your gratitude by doing something for them in return. The more personal the better, the more thought you put into it, the more the recipient will understand how much you care. The simplest action, if well thought through, can mean more than the most elaborate procedure that doesn't fit the situation.
And if you do use words, consider expressing your gratitude just saying 'thank you'. If you tell someone what their actions meant to you or how they made you feel, it will mean so much more than just saying thanks.
If we believe that fae are creatures of truth and trickery both, that they may be bound by their words and yet free to twist them to deceive, it only makes sense that they might distrust thanks. Add in the concept of owing a debt when someone does something for you and you acknowledge it and you begin to see how the stricture against thanking a fae would prevent you from being bound to pay them back in whatever way they saw fit.