For me, thankfulness is something of a strange concept. Maybe that's because I'm not religious and so I've never been into revering any deity's supposed kindness towards me. As for nature, that has no will to make me happy. Other people? Maybe, but I'm not sure. I think there's a lot in this theory that people help each other out for ultimately selfish purposes, such as group survival or the hope of the favour being returned.
I enjoy helping other people out from time to time, alleviating their pain, offering them the means to solve a problem they have, but I don't expect a lot of praise for it, and I don't see it as me being altruistic or virtuous, I just get a buzz out of doing that kind of thing, which is its own reward. I like to be thanked, but only briefly. I don't like it when I help somebody and then they either never help me back or have the nerve to attack me later. And I tend to expect others to feel the same when they help me. So I'll say "thanks" at the time and then it's discharged, just like saying "sorry" when they've annoyed me. Just plain courtesy really.
Actually the whole idea of saying "thank you" got rather tarnished when I was a child, because of certain adults who insisted on it. As for being "thankful," if that means being full of thanks, I'd reserve that for occasions when somebody had done me an enormous favour.
So, when has that happened to me? My parents did harm to me as well as good, so overall I don't remember them as people I should be very grateful to, though they had their moments. My early years would likely have been a lot worse without Dad's help. There was a slightly older schoolboy who beat up a bully who was about to beat me up. I never saw him again but I remember him with gratitude. There have been a lot of little things like that here and there in my life. Sometimes they were unintentional, like when I've learned from somebody by watching how they handle a situation. Don't know if it counts if they were unintended favours. I suppose they do.
Summary: Being grateful exists in me, but it's no big thing, and I wouldn't want to set aside a special day for feeling it intensely. I don't really do special-day observances. To me it's obvious that a modicum of gratitude is important. I don't need to be especially reminded to stop being ungrateful because I'm not particularly ungrateful.