You must be the change you wish to see in the world.
Once when we were passing each other Swamiji planted himself in front of me and said "You should be happier." I replied "I'm sure you're right, Swamiji. Almost everyone—" Swamiji cut me off: "No! No! YOU should be happier!"
Related post: http://martyrion.blogspot.com/2012/09/my-dharma-is-penance.html