Saying 「thanks」 and expressing gratitude are good for health
Any parent raising a child knows how much time is spent on teaching that child some manners. Say "thank you" when someone gives you a present. Say "thank you" when somebody holds the door. Say "thank you" when the server notices you left your favorite binkies under the booth and rushes out of the restaurant to get it to you.
As adults, we surely don't need reminders for the typical thank you moments. But many of us may need reminders to do so beyond the typical door openings and gift receiving; after all, part of our purpose here is to get a little deeper, right? Once a week (or more often as you enjoy it more), think of someone who had an effect on your life - big or small - and write that person a note of gratitude (not via e-mail either; be personal).
Gratitude is one of the gifts you can give others that also has some selfish benefits: Some research shows that 15 minutes of daily gratitude can dramatically decrease stress hormones in your body.