Two of the most beautiful words to roll off a submissive tongue, or ping up on my phone. Those two words sweeten my feelings towards those under my charge, making me smile as I recall (or imagine, if I'm not present) the occurance that I am being thanked for. The act of saying thank you places me in a position of power, instantly, like a sugar rush. I am a benefactor, a gracious giver of sensation. I've actually recently started to use the word patronage to describe my leverage as a dominant, it expresses the exchange in a way that has pleasing reminders of lords and ladies keeping valuable artists, perfected servants and objets d'art. It also emphasises the fact that dominance is not all about taking, or even about simply doing, what I want without consideration, there is method and reasoning in what I am doing. I offer something very valuable which cannot be taken for granted.
Gratitude, when truly felt, is a surprisingly complicated emotion, relying on an understanding of the value of what is being given and the cost of giving it. There's a balancing act to be struck with using it - rather like apology it can be overdone (I have a pet hate of people who say sorry too often and try to drum it out of those around me). The trick is to use it sparingly and at specific points that are outside of the normal framework for when that person might use that word.
I require politeness in general, but messages of thanks form part of my stated protocol with my pets. I want them to be reminded of our relationship at certain key instances - orgasm is a particular favourite, but equally after punishment or an order is issued. I'm deliberately framing and highlighting the nature of our power exchange, that I am responsible for both the pleasurable and the painful parts of our sexual interactions and that they should feel grateful for it.
It can form a useful training tool not only because repeated actions and phrases can help ingrain a sense of structure but because the act of thanking someone establishes a bond between two people. The more genuine thanks that are offfered, the more connected we become. There are other, alleged positives in even the act of saying thanks itself. There have been various studies which show that gratitude can act as a mood enhancer, improve mental health and even improve sleep. Whilst I'm tending towards taking that with a pinch or five of salt I do like the idea that I could make the lives of my submissives even better by getting them to feel even more grateful towards me - it certainly sounds like a virtuous circle worth working towards.