Go on, be happy, be adventurous – take some exercise.
Fitness = happiness! Due to the excitement and adventurous challenge within exercise and movement, our bodies create and release the happiness molecule – better known as serotonin.

More: http://www.mailonsunday.co.uk/health/article-2099133/Happiness–hitting-gym-Exercise-fans-feel-greater-levels-excitement-say-scientists.html