Fulfilment of desires generally leads to happiness.
Most of the times we need to accomplish that ourselves, however, there are cases where it can happen without our active pursuit.
I usually feel a sense of general contentment with my life. My life is awesome. First world country and all, I have had the chance to be able to pursue my happiness, and have it mostly be in my control.
If you follow Maslow's Hierarchy of needs (Which I am doing in my definition of 'happiness' just for simplification purposes), some people only need the bottom part satisfied in order to be happy, because to them, that is what is attainable. You can make your happiness by reaching and achieving goals, which leads to a sense of self-respect and worth, or you can live life day to day without direction and still be complacent. I don't really think you end up completely happy to your fullest ability.