"There is no path to happiness: happiness is the path." — Siddhārtha Gautama
There is no more important thing a person can learn than that happiness is intrinsic. It is not the resulting effect of some other action or of a certain circumstance that you have wandered in to. It is not brought to you by others or kept there by following a set of rules laid forth for you.
Of course, there are things that can happen that will harsh your mellow. It's not as if people suffering disease or living in war torn countries can choose to be happy and the bad stuff goes away. I'm not a self-help guru with delusions of all the ills of the world being solved by a smile and some well-formed motivational sentences.
However, there is something to the idea of choosing to be happy when that choice is yours to make. Those of us living the privileged lives of the first world are most often able to make this choice for ourselves. Factors like mental illness, life pain and other baggage can bring you down but in the end the journey back to happiness starts as soon as you decide you are happy and that you shall accept no less.
I love the idea of a happy moment causing a smile and a smile causing a happy moment. They are interlinked and both are cause and reaction.
Be happy by making the decision to be happy :o) Trust me, it works.