There is no such thing as gratitude unexpressed. If it is unexpressed, it is plain, old-fashioned ingratitude (Robert Brault).
Whenever we Thank someone or someone Thanks us, it gives us a sense of fulfillment that we made someone happy whether its someone we know or could be a total stranger.
Taken from Google
I couldn't find its origin on the internet as most search results were also blog entries from people from other parts of the globe (I hope someone would give me a link tom where it all began). Also, I'm not sure if some of the blogger's page on facebook knows about this event. Knowing our culture and our race, its customary for us to say Thank You whenever a good deed is done to us or vice versa.
Anyways, I say Thank You for reading and Thank You! :D