Being kind can trigger a release of the hormone oxytocin, which has a number of physical and mental health benefits. Oxytocin can significantly increase a person's level of happiness and reduce stress levels. It also protects the heart by lowering blood pressure and reducing free radicals and inflammation, which incidentally speed up the aging process.
I love seein science back up age-old beliefs like "Do unto others as you would have them do unto you."
(H/T to @ddraper)