"Thank you" is social etiquette. I teach my children how to have manners & thanking others who do something for you, a service, a gift, a kindness, is part of that etiquette. I am not always thankful for what has been done, but I am thankful for the gesture, as no one generally has to show you a courtesy of any sort. I also teach my children to say "hello" when spoken to (unless they have a bad vibe for some reason) to say please when making a request, to say pardon me before interrupting a conversation or physically maneuvering around a person. These are social skills and teaching social skills can only help your children navigate the world a little easier. If they are perceived as rude, it will make their lives harder for it.
That said, I teach my children to say thank you from the time they can talk and later I teach them about what it means to have a gracious spirit and to appreciate all things, from having a comfortable bed to enough food to eat and often with options we enjoy, and for the gestures of others, offered in kindness. If you teach a child gratitude, you need not worry to "force" them to say thank you, but when young they might occasionally need a reminder of the appropriate polite response, which in my culture is "thank you". I've never needed to demand my kids say it, but I also work on making sure they understand gratitude on the whole. Having a grateful heart can only enrich your life and help you find a sense of contentment and happiness in all things.