The definition of happiness varies from one person to another. But in its essence, happiness (just like everything else) has a lot of science behind it. From the food you eat to the temperature you're in, there are a lot of factors that come to play when deciding how you feel and determining your mood. So it's probably a good idea to understand these factors to achieve long-term happiness.
This eye-opening infographic developed by WebpageFX sheds some light on the science behind being happy: