What is HAPPINESS?

I've been thinking about this a lot. Who is really and truly happy? What are those qualities that are common to happy people? I'm not talking about fake / always-idiotically-grinning / annoyingly cheerful people. I'm talking about those who weather life's storms with a calmness that makes them stand out, those who make their decisions with quiet yet strong confidence, those who don't start crying or freaking out over small every-day annoyances. Those who are... serene... maybe. Although serene is a weak word - it assumes that the person is quiet and passive. Is happiness simply ignoring the problems and going with the flow? Is happiness just accepting your lot in life and not look too much into your own soul to see what's missing? Is happiness finding a cause and throwing yourself into it to the extent that everything else will seem unimportant? Certainly, zealously religious people seem to do this and they seem happy. Activists, volunteers, those who do stuff for their community seem happy.
And, I thought often: do I want too much? I want a job that fulfills and makes me feel like I'm doing something important. I want a husband who is home and there for me in every sense of the word, a true life partner who I can talk to and love and be loved by. I want a kid who is not so stubborn. I want a mother who listens and doesn't try to preach at me. And yet, how many people in this world are lonely, childless or orphaned? And, if I would have all those things, would I be finally happy? Or would I then look for more alone time, privacy, a more assertive child, a mother who is not so involved in my life?
I have a friend who divorced a couple of years ago and she is now in a new relationship. Whereas her ex was aloof and distant and commitment- and closeness-phobic, this new man can't get enough of her presence and she finds the constant attention suffocating.
If there is such a thing as "happy medium", how come it's not in the same place for everyone? How can we find it, capture it and keep it, so it stays with us always?
I never had a problem finding happiness in moments of time... but I'm unhappy with my life overall, and yet too scared to change it. How do I get past the fear? Is being fearless equals being happy?