I've always been shy, and saying thank you used to be the hardest thing for me. I'd always think hard about saying it, how I needed to say it, but I'd freeze up and get scared that it would sound stupid or even sarcastic. I was convinced I'd come off as less rude by not saying anything.

I don't even remember how, but I eventually trained myself to say thank you so automatically that now I probably overuse it a bit. But it's not always the easiest thing for even a shy person to say.