Gretchen Rubin describes in her post How To Be Happier – in Four Easy Lessons the series of truths she came up with, based on her study of Buddhism.
Her Fourth Splendid Truth is “You’re not happy unless you think you’re happy. Corollary: You’re happy if you think you’re happy.”
One of her books: Happier at Home: Kiss More, Jump More, Abandon a Project, Read Samuel Johnson, and My Other Experiments in the Practice of Everyday Life. http://vsb.li/EmdyZn