Banal, boring and uncreative answer.. but that's what I would have said too. Sometimes simple answers works the best.
It's actually one of the hardest questions to answer. You can answer it with a long, long post, full of deep thoughts but it all comes down to a couple of things: being healthy, happy and surrounded with people you love. Most of the time we're not bound to pick the first option, sadly, but other two are possible.
Then again, even if many people do agree with that, I'd say not many agree with the definition of happy or what you do in order to be in that state. Too many wrong paths, yet most of the people pick those.