Last week was Thanksgiving in this land. This week should be also. And next week. Every week.
While words (“Thank you” and “I really appreciated that”) and acts (reciprocating when someone does something nice for you) of gratitude are known to provide mood boosts (especially for the person expressing gratitude), they must be repeated for the boosts to last. The technical term for this is hygiene. Hygiene is the practice of cleanliness for the purpose of preventing illness—washing hands, brushing teeth, washing hair, and so forth. It is the nature of hygienic activity that it only lasts a little while and must be repeated periodically in order for its effect to continue. You have to brush and floss every day, not just once.
That is why some say that gratitude has a hygienic effect on mood. To write a thank you note today leaves you with a feel-good glow. But not for long. Tomorrow, you need another act of giving thanks to continue, or rekindle, the glow. Brush your teeth, and give your thanks away daily.
Read more about the hygienic effects of gratitude, and also of altruism/service in general, in the works of University of California, Riverside, psychologist Sonja Lyubomirsky, such as The How of Happiness (2008).