Every year on Thanksgiving, we are called to give thanks for the good things we have... and of course, gorge on lots of food. I'll resist the urge to write about overeating and focus instead on what the holiday is really about: gratitude.
Though in the past, the field of psychology has focused on maladaptive behavior, the expansion of various subfields of psychology beyond clinical psychology has led to the study of a variety of behaviors, both good and bad, and to focus on, not only the things that make us mentally ill, but the things that make us healthy, happy, and fulfilled. The study of gratitude is one area studied by so-called positive psychologists.
There are certainly individual differences in ability to feel gratitude; some people are simply more grateful than others. (You can find out more about your "trait gratitude" by taking this measure). But social situations, like Thanksgiving, can also influence your minute-to-minute levels of gratitude (or "state gratitude").
Gratitude, unsurprisingly, is strongly associated with psychological well-being, happiness, and life satisfaction
(find two full articles about this here and here). Feeling gratitude reduces stress and positive coping; these positive benefits are observed even when people are randomly assigned to an intervention meant to increase their gratitude (read one such experiment here), meaning that these benefits can be reaped by anyone, not just people are are "naturally grateful".
Being the target of gratitude is also beneficial. Being told "thank you" makes a person more likely to repeat the behavior in the future, probably because it functions as a reward (and as I've said before, if a behavior is rewarded, it's more likely to occur again). So even if you feel like someone is "just doing their job", saying "thank you" can make him or her feel more motivated to repeat that behavior in the future and will likely improve your future interactions with that person, as well.
So keep feeling that gratitude, today and everyday - it's good for you. Happy Thanksgiving everyone!