I often think a lot of wise things in my head. To be honest, I think I am the nicest person on earth, in my mind and in my head. Often I receive complaints from my wife about things that I have forgotten to do and things I haven't said.
That might be cruel, especially when I know that I had the thought in my head. But, when I tell her, it doesn't help a lot. Do you know why? Even though you have a good intention, or if you have a brilliant idea... it doesn't help a lot if you do not put it into action or let the thought leave your mouth.
In the same way, silent gratitude isn't of much use to anyone. That is so true! You might be really grateful for something, but if you do not let the person know that you are grateful, they will not know about it, and they might skip doing such things for you in the future... after all, you weren't happy or grateful at all!
What is the conclusion?
You should rather say thank you too often, than not say it at all!