From WebMD:

Happiest people were 22% less likely to develop heart disease over the 10 years of follow-up than people who fell in the middle of the negative-positive emotion scale.

People with the most negative emotions had the highest risk for heart disease and people who scored highest for happiness had the lowest risk.

Possible explanations for how happiness may protect the heart:

- Healthier lifestyle: Happy people tend to sleep better, eat better, smoke less, and get more exercise.
- Physiological impact: Happiness may produce a host of positive chemical changes -- such a reduction in stress hormones.
- Genetic influences: It could be that people who are predisposed to happiness are also predisposed to have fewer heart attacks.

Devote 15-20 minutes a day to doing something enjoyable and relaxing.

Strategies that could help naturally negative people become happier:

- Express gratitude on a regular basis.
- Practice being optimistic.
- Engage in frequent acts of kindness.
- Visualize one's best self.
- Savor joyful events.
- Practice forgiveness.

Regular exercise, sexual activity and good sleep are associated with increased self-reported happiness.

References: