"Happiness is a perfume you cannot pour on others without getting a few drops on yourself." - Ralph Waldo Emerson
In undergrad, I took a sociology class. I remember during the lesson, the professor made a bold statement that was up for debate, "everything you do in life is selfish." I disagreed. How could community service be selfish? How could helping others be selfish? How could these acts be selfish when you're doing for others? The truth is, they are selfish. You like how you feel when you do such positive things, that you continue to do them. Therefore, friendships are selfish because you like how you feel when you are with those people. You like how you feel when you are doing good things and helping others.
Happiness is the same way. We do things because they make us happy, but in return we are making others happy. It is difficult to say hi to someone and them not to respond with a smile. A natural response is a positive one. I challenge you to smile to others, pour out some of your happiness perfume.