It's different for each person. I, myself, am very happy. I have everything I need, I have good career prospects, I have a loving family, I live in a stable, peaceful, highly developed and egalitarian nation (Australia). I have never been the victim of any crime, I'm not addicted to any substance, I'm in almost perfect health (I have teeth problems, but they're minor) and I've had a great education. I'm insanely lucky.
However, there are people for whom life has been nothing but misery - just look at the dying kids in Somalia - living in a war torn nation, constantly in danger, no educational prospects outside being brainwashed by militants (not exactly an education), no real career prospects other than joining the militants, no law, no order, and at anytime your family members could be killed. And on top of that, you're starving to death, and your own people are keeping aid away since it comes from "the infidels". They are insanely unlucky.
I love my life. My life is (so far) great. Of course, every life has ups and downs. No matter how happy you currently are, your life is fragile. I could be diagnosed with cancer two months from now. My parents could be killed in a car-crash and there's nothing I could to do stop it. War could suddenly break out.
So while life is inherently uncertain, and while it has good and bad parts, I'm going to focus on the positive things in my life and enjoy them while I can. I know that one day my good fortune will come to an end (nothing lasts forever), but so far I've had 24 years of good fortune, and for that, I think I've had a good life. Whatever happens next could totally suck, but it can't erase the good memories I've accumulated (unless I suffer from a massive stroke which destroys my memory. Which could happen).