1. Family and Siblings - A broken family and the only child combo is by far the worse you can ever get. If you are in a happy family and having strong bonds with your brothers and sisters, you have a good start in life. Unfortunately, this most important factor is not within our control. You are born with it.
2. Marriage - This you can control. Many research shows that married couples are happier than singles. You have someone that you can trust, can talk to, can relate to, can enjoy activities together and so on.
3. Friends - Being social is being happy. Humans as we know are very social creatures. Loneliness and boredom are two main factors that contribute to sadness. Thus, having more CLOSE friends = more happy. You have no time for loneliness and boredom. Fortunately, this is well within our control. Introverts have a small disadvantage in this aspect. However, we can definitely try our best to be friendly and be genuine in our friendships.
4. Career - Part of how career contributes to happiness is money. However, the more important part is that a good career takes away our boredom and loneliness. You gain a sense of purpose in life. You felt that you did something productive and being useful. Too much free time such as those unemployed leads to stray thoughts and loneliness and boredom.
5. Religion - Just a tool for us to be more social. We get to know more friends which relates to point 3. We think that we have a sense of purpose in life. We have something to do in the weekend. Even a cult could serve the same purpose. Just look at how people got high and happy in mega church workships. Basically, it is an additional commitment that we chose to take up to eliminate our loneliness and boredom.
6. Money - Once you have all the basic ingredients in place, then money comes in to boost your baseline happiness further. If you lack of the above basic ingredients, no amount of money can truly bring you happiness in life. Money is merely a booster. Think of it like alcohol which helps to boost your experience in a party or social events.
So in the end, to sum up, a person can be said to be one of the most unhappy people in the world if he or she came from a broken family and have no siblings, perhaps leading to introvertedness, has fewer friends, has no religion and no purposeful career or nothing major to do in life.
Buddha is totally of a different league. He doesn't seem to require socialness to be happy. How Buddha stays happy in his own solitude, in his own mind, is another story to be told.

