I guess saying thankyou, Jazakhallah khair,shukran, are all those very easy to say phases that can bring the receiver much happiness. Nothing like feeling appreciated! It can help foster better relationships and friendships, increase love and understanding, better a person's capabilities and productivity, and in general do so much good. Its very unfortunate then that we neglect to just say it to those we love, appreciate and value. 

Lastly a reminder :-""He who does not thank people does not thank Allah."

(The Prophet Mohammed)