Creates a smile even on the worst days
One of the best ways to make the people around you happy. You want your friends, co-workers, family, etc. to be happy right? Say it and MEAN it
Thank you gets its powers from scarcity
"Thank you" has such a strong effect on us because you have to earn it. Those words do not just appear from being idle. It's a reward for being a great friend/co-worker
Thus you need to find a balance
Reward the people who deserve it but don't devalue your "thank you"s by using it too much.
Overusing it falls on deaf ears
Thank you is no longer special when you over use it. Err on the side of overusing than underusing, but there is an upperbound as well
Thank you isn't a simple word.
But very important