I'm glad you asked this Anthony! I happen to have a lot of knowledge in this area.
Commonly, people will say that "attitude" plays the biggest role. However likely this is wrong. CIRCUMSTANCE A.K.A Environment is the most powerful factor. Genes are nothing more than the levels you'll be born with.
Attitude however is the most valuable asset for creating new levels of happiness. There are several simple ways to utilize this concept to achieve complete happiness, this is the basis of a popular newsletter I own.
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Hope I've helped.