You ask a very broad, almost unanswerable question. So, rather than tackle it myself, I'm going to suggest you read a remarkable book on the subject, The Art of Happiness, by the Dalai Lama and Howard C. Cutler (a psychiatrist here in Arizona). The book is based on a series of conversations between the two men.
The opening line comes from an address by the Dalai Lama: "I believe that the very purpose of our life is to seek happiness." He goes on to examine the elements that go into making happiness possible: good health, the wealth or material things we accumulate, and fulfilling relationships. However, His Holiness explains that none of this will produce happiness if your mental attitude is negative. "If you harbor hateful thoughts or intense anger somewhere deep down within yourself, then it ruins your health...if you are mentally unhappy or frustrated, then physical comfort is not of much help."
His Holiness is right - anger has real health consequences. A new study, published in the medical journal Circulation, found that a person who is prone to anger is about three times more likely to have a heart attack or sudden cardiac death than someone who is not as angry.
The Dalai Lama adds, "As long as there is a lack of the inner discipline that brings calmness of mind, none of the external facilities or conditions you have will give you the feeling of joy and happiness that you are seeking."
Those words of wisdom may sound simplistic, but in reading the book you will find a fascinating and nuanced discussion of what happiness is and how we can all achieve it. Cutler has amplified the Dalai Lama's philosophy with findings from scientific studies, as well as cases from his own practice. For example, after a discussion with His Holiness about the differences between happiness and pleasure, Cutler tells of a patient who couldn't decide whether to move from a hot, crowded city to a small town that she loved. The problem was that she loved her job in the city and didn't think she would enjoy the one she had been offered in the small town. Cutler asked whether the move would bring her happiness or pleasure. That question made the decision easier for her - the move might bring her pleasure, but she decided she would be happier if she stayed in the city.
This book can give you an entirely different outlook on happiness and how to achieve it. I recommend it highly.
Andrew Weil, M.D.